Basic Cave Diving Terms

A Dictionary of Cave Diving Terms

compiled by Matt London


Adaptation — the process of making adjustments in response to the environment.
Abseil – A controlled descent of a rope using friction obtained by wrapping the rope around the body in a particular way or passing the rope through a karabiner.
AGE — Arterial Gas Embolism — Entry of gas emboli into arterial circulation. Breath holding is most common cause.
Al 80 — Most common tank in industry. Aluminum 80 – capacity 11 liter.
Anastomoses — small, winding tubes that interconnect with each other in a maze-like pattern, normally along bedding-planes; frequently seen on ceilings where the rocks below have fallen away.
Anoxia – A total absence of oxygen
Aquatic — describes a surface or underwater habitat and the animals that live in it.
Aquifer — a zone of the earth, rocks and/or sediment able to transmit useful amounts of ground water.
Arrow Markers — Arrow markers are placed on the line to indicate the most appropriate direction of exit from the cave. It is recommended that permanently placed arrow markers should indicate the distance (in meters) to the cave exit and be positioned on the line at all intersections, on the outgoing side of the line indicating the exit direction. Arrows at intersections should be placed a short distance from the intersection to avoid confusion with other lines, but preferably within easy physical reach.
Artifact — an object produced by human workmanship.
ATA — Absolute pressure. Includes the atmospheres of pressure you accumulate under the water and the atmosphere of pressure at the sea level.
Azimuth – The true bearing of a survey line, determined by measurement from an accurate survey.


Barotrauma – pressure injury.
Bat — the only mammal that can fly, known for its use of echolocation to move and hunt in the dark.
Bat Gate — a gate constructed at the entrance of a cave or mine, designed to prevent humans from entering while permitting free entrance and exit of bats, other creatures and natural airflow.
Bed — a layer in rocks.
Bedding Plane — the surface between two contiguous layers of rock.
Belay – 1. To attach to an anchor. 2. To operate a safety line.
Blue Holes — cave surface openings on the ocean floor, can be prone to reversing currents with tidal flow.
Body Fat — Fat absorbs about six times as much nitrogen as an equal weight of blood (nitrogen reservoir for bubble growth). Fat’s high nitrogen solubility increases absorption and bubble growth. Fatty tissue’s excess accumulation of inert gas (nitrogen) can greatly slow elimination. Statistics show that the fattest 25% of the diving population, as measured by skin-fold thickness, have a ten-fold increase in DCI incidence.
Bottom Mix — Mix breathed at the bottom, especially when other gas mixes are used for travel and/or decompression. Bottom mix should have ppO2 less than 1.4. END for TCDP dives is typically set at maximum 27 meters!
Bottom Time .— The total elapsed time from when the diver leaves the surface in descent to the time (next whole minutes) that he begins his ascent, measured in minutes.
Boyle’s law – “For any gas at a constant temperature, the volume of the gas will vary inversely with the pressure.
Braille Dive — Low visibility dive. i.e. to feel your way around.
Breakdown — a large or small accumulation of rough jumble of angular rock fragments filling all or part of a cave passage after the collapse of part of the walls and/or ceiling. Also can form into a “debris cone”.


CAGE – Cerebral arterial gas embolism. Bubbles of air that obstruct the arterial blood flow to or in the brain.
Calcite — a mineral (CaCO3) composed of calcium carbonate; the principal component of limestone.
Call the Dive — “thumbs up” command signal for finish the dive or start back to surface.
Calcium Carbonate — a compound (CaCO3) found in nature as calcite; in shells and used in making lime and cement.
Carbon-14 Dating — the process by which the radioactive decay of the carbon-14 isotope is measured to determine the age of organic (once living) items. Most useful for objects 100 years to 70,000 years old.
Carbon Dioxide — often found in dry cave passages and air pockets. Normally displaces oxygen.
Carbonic Acid — a weak acid (H2CO3) formed by water reacting with CO2, especially in the soil.. This acid can dissolve limestone, and is the primary agent in the creation of solution caves.
Carboxyhemoglobin – Hemoglobin that has bonded with inhaled carbon monoxide.
Cartography — the science and art of making maps. Modern cave divers survey caves as they explore them and later draft maps from these surveys.
Cave — a natural underwater chamber or passage big enough to be entered by divers. v. to explore a cave.
Cave Coral — a cave formation consisting of small knobby clusters. See ‘popcorn’.
Caver — a person who explores caves. Less commonly known as a spelunker.
Cavern — an natural underwater chamber often of large size that sunlight can enter and illuminate.
Cavern Dive — A dive performed inside the cavern zone and within direct sight of the surface entrance conducted in daylight hours only and permitting emergency ascent (40 linear meters) of the surface
Cavern Zone — also known as daylight zone. Within sight of surface entrance and in the sunlight.
Cave Diving Hazards — General hazards include total darkness, loss of visibility, increased potential for confusion, unexpected currents, cave ins, restrictions, depths and unknown distances.
Chiroptera — “hand-wing;” the scientific order that bats belong to.
Clay — Classification of very fine silt (includes anything smaller then 1/256 mm).
Clay Bank — sediments laid down in beautiful layers sometimes forming banks or walls. Very fragile and can be ruined forever by just one careless diver!
Cockpit Karst – Conekarst in which the residual hills are chiefly hemispheroidal and the closed depressions often lobate.
Coffin Cave — Cave used as a burial site.
Column — a cave formation (speleothem) formed when stalactites and stalagmites grow together, or when one of them grows all the way to the floor or ceiling. Forms when the cave was dry.
Commercial Cave — see ‘show cave’, sometimes referred to as sacrifice caves
Cone Karst – Karst, usually tropical, dominated by its projecting residual relief rather than by its closed depressions.
Conduit – An underground stream course completely filled with water and under hydrostatic pressure.
Confining Layer — layer of clay or other non permeable non soluble type sediments. Effectively restricting or putting a ceiling on the Karst resource.
Coral Caves — caves formed by coral growth. Hazards include dangerous aquatic life (sharks, eels, stonefish etc..), sharp coral, that becomes more of a concern with waves or surge and tides.
Curtin – A speleothem in the form of a wavy or folded sheet hanging from the roof or wall of a cave, often translucent and resonant.


DCI — Decompression sickness (DCS) or alveolar rupture, with resulting bubbles in the arterial circulation (arterial gas embolism [AGE]) and all of their manifestations are now normally grouped together under the heading “decompression illness” (DCI).
Deco — process of controlling body tissue off gassing and bubble growth thru a series of stops performed during final ascent. Decompression Schedule. A listing showing required decompression stop depths and stop times for a particular depth and bottom tome normally indicated as feet/minutes.
Decompression Schedule — A listing showing required decompression stop depths and stop times for a particular depth and bottom tome normally indicated as feet/minutes.
Decompression Stop — Specified depth at which a diver must remain for a specified length of time (the stop time)
Depth – When used to indicate the depth of a dive, means the maximum depth attained by any part of the diver during the dive, measured in meters of seawater.
DIR — see Hogarthian
Debris Pile – A heap of blocks in a cave, roughly conical or part-conical in shape.
Decoration – Cave features due to secondary mineral precipitation, usually of calcite. Syn. speleothem.
Disappearing Stream — in a karst region, a river or stream that flows into a sinkhole or crack and from there into an underground or cave river system. also see siphon.
Discharge Zone — the area where water emerges. In a cave, the discharge of a spring where ground water emerges as surface water into a stream, lake, or ocean.
Dissolve — to cause to pass into solution, to separate into component parts. Carbonic acid dissolves limestone by separating the calcium and carbonate and creating a liquid.
Dive Profile — A table or graph of time/depth coordinates for an entire dive showing all desired stop without regard to decompression obligation.
Doline – A closed depression draining underground in karst, of simple but variable form, e.g. cylindrical, conical, bowl or dish-shaped. From a few to many hundreds of meters in dimensions.
Dolomite — A mineral consisting of the double carbonate of magnesium and calcium, CaMg(CO3) . Caves such as those found at Oregon Caves National Monument in the USA are formed in dolomite. Tends to be more yellow in color.
Dome — a vertical shaft in a cave as viewed from the bottom; formed by water dripping or flowing straight down through vertical cracks. See pit.
Down-Stream — swimming or moving with water flow or current.
DPV — diver propulsion vehicle, also see “dive scooter
Drapery — a speleothem which forms when drops of water run down along a slanted ceiling; also known as “bacon” by some dry cavers.
Dripstone – A deposit formed from drops falling from cave roofs or walls, usually of calcite.
Dye Tracing — the process used by scientists to track the path and speed of water through a cave. Environmentally safe dye is put into streams and sinkholes; then water in the cave and at discharge areas is tested for the presence of dye and noted for the speed at which the dye moved through the cave system or establishing a relation between cave systems.


Echolocation — the ability of an animal to orient itself by receiving the reflection of sounds it produces, such as with bats and dolphins.
Ecology — the study of the interrelationships of organisms and their environments.
Endemic — a plant or animal native to a specific area and habitat and found no where else. Many fish, shrimp and other organisms are endemic to specific caves in Thailand. Environmental encroachment and development poses grave threat to these resources!
END – Equivalent Narcotic Depth. The depth at which AIR (21% 0xygen/ 79% N2) would cause as much narcosis as a given helium based gas mixture and depth. Any narcotic potential of helium (He) which is small, or perhaps even non existent is discounted and the exact narcotic effects of oxygen still remain somewhat of a mystery. This formula is based on nitrogen content only:

END = [(FN2 ÷0.79) x (Depth + 10 msw)] – 10 msw

Endangered Species — an animal or plant species whose population has decreased to the point where it is in danger of disappearing forever.
Epsomite — a magnesium sulfate mineral (MgSO4•7H2O), usually white, colorless or gray. Epsomite is valued for its medicinal properties; commonly sold as “Epsom salts.”
Erode — to wear away by the action of water, wind, or glacial ice.
Erosion — the action or process of eroding. While weathering separates materials and breaks them down, erosion transports materials to a new location.
European System — Primarily just a name devised by American cave divers to describe guideline placement systems common in cave system throughout the Caribbean with no distinct passageway configuration. Typically example: Blue Holes of the Bahamas with “lines going everywhere”.
Evaporation — changing of liquids into a gas.
Exsurgence — water emitting from a cave supplied almost entirely by seepage waters from karst recourses.
Extinction — the process by which an animal or plant that once existed can no longer survive and then can not be found alive anywhere on earth.
Extirpation — the process by which an animal or plant that once lived in a region can no longer survive in that region and can no longer be found there; usually due to hunting, habitat loss, or pollution.


Fauna — animal life; especially the animals characteristic of a region, period, or special environment. In this case the cave environment.
FFM — full face mask.
Flint — a hard smooth rock ranging from white to black often found in layers of limestone. This rock often used for tools and as a fire starter
Flow — current created by cave spring discharge.
Flowstone — a speleothem formed when water flows down walls, over floors, or over older formations.
Food Chain — a series of plants and animals linked by their food relationships; for example: a plant, a plant-eating insect, and an insect-eating bat would form a simple food chain.
Fossil — a remnant, impression, or trace of an animal or plant of past geologic ages that has been preserved in the earth’s crust.
Frog Kick — Propulsion technique favored by cave divers to avoid silting.
FHe — Fraction of helium (He) in a mix. In air, fHe = 0, unless you are near certain natural gas wells, a birthday party, or tech divers.
FN2 —. Fraction of nitrogen (N2) in a mix. In air, fN2 = 0.791 = 79.1%. Actually this is a lie, counting the approximately 1% of other gasses in air (noble gasses etc.) as though they are nitrogen, but it is close enough.
F02 —Fraction of O2 in a mix. In air, fO2 = 0.209 = 20.9%.


Gap Spool — used to join the ends of two permanent lines. Jump spool is used when leaving the middle of one line to physically connect with another line.
Gas Diffusion — Another physical effect of partial pressures and kinetic activity is that of gas diffusion. Gas diffusion is the process of intermingling or mixing of gas molecules. If two gases are placed Together in a container, they will eventually mix completely even though one gas may be heavier. The mixing occurs as a result of constant molecular motion.
Gas Laws — Gases are subject to three closely interrelated factors: temperature, pressure, and volume. As the kinetic theory of gases points out, a change in one of these factors must result in some measurable change in the other factors. Further, the theory indicates that the kinetic behavior of any one gas will be the same for all gases. Consequently, basic laws have been established to help predict the changes that will be reflected in temperature, pressure, or volume as the conditions of the operating environment change. As a scuba diver you need to understand what effect changing pressure will have upon the air in your dive equipment, exposure suit and lungs as you move up and down in the water.
Gay-Lussac’s Law — “for any gas at a constant volume, the pressure of the gas will vary directly” The pressure-temperature law, which explained what would happen if a scuba cylinder of a certain volume were pressurized to a specific pressure and then heated. The pressure inside the tank will increase.
Geology — the study of the history of the earth and its life, especially as recorded in its rocks.
Glacier Cave – glacier caves are formed by the melting of ice near the leading edge of the bottom of a glacier. These caves, while scenic and unusual, are considered to be among the most dangerous and unpredictable for exploration. The dry sections of many of these caves have been explored for miles, testifying to their extensiveness. The underground rivers found in glacier caves feature variable current based on time of year and other environmental factors.
“Golden rule” – In cave diving – Anyone can call the dive for any reason at any time!
GPS — Global Positioning System
Groundwater — water that infiltrates the soil and is stored in slowly flowing reservoirs (aquifers); used loosely to refer to any water beneath the land surface.
Guano — the rich manure of bat dung.
Gypsum — a sodium calcium sulfate mineral (CaSO4•2H2O), colorless, white, or yellowish, found in powder or crystal form. Used for plaster, cement, and medicinal purposes; precipitates from the walls in the form of gypsum “flowers.”


Habitat — the place or type of site where an animal or plant naturally or normally lives and grows; the arrangement of food, water, shelter, and space suitable to an animal’s needs.
Habitat for Decompression — small room or cubical fixed at a specific in water location and depth used to increase diver safety and comfort while maintaining maximum decompression oxygen window or gradient during longer shallow decompression stops.
Heliox — Breathing mix comprised of helium and oxygen.
Halocline — separate and distinct layers of water frequently found in coastal caves. The layers are stratified due to density differences. i.e. salt water is denser then fresh so may tend to be found more at depth.
Half -Time — A half-time is the same as a half-life when considering radiation. A half-life is the time for a radioactive sample to decay to half its original value. Half-time tissues are named for the time in minutes it takes to fill (saturate) half the tissue with nitrogen. Then, it takes the same amount of time to absorb enough nitrogen for the other half to become half saturated. Then, it takes the same amount of time for the quarter that’s left to become 50 percent saturated, then, the same time to absorb enough for the eighth that’s left to become half saturated, and so on.
Hand Signals — must be one handed in cave diving, so you can hold a guideline reel in the other.
Helictites — speleothems that grow as small, twisted structures that project at varying angles.
Hemoglobin – Molecule in the red blood cells that transports oxygen.
Henry’s Law — Henry’s Law states: “The amount of any given gas that will dissolve in a liquid at a given temperature is a function of the partial pressure of that gas in contact with the liquid and the solubility coefficient of the gas in the particular liquid.”
Hibernacula — places where bats or other animals hibernate, or sleep, during the winter to conserve energy.
Hogarth Rig – A method of rigging scuba gear for maximum survivability.
Hogarthian Principles – Principles behind Hogarthian system originated for cave diving but now adopted more widely. See Why right doc.
Hydrogeology — science of movement and occurrence of subsurface waters and with related geologic aspects of surface waters.
Hypoxia – Low oxygen at cellular level


Inert Gas — any gas not metabolized by the body.


Joseph Priestley – discovered Oxygen in 1774 .
Jump Spool — line spool attached or “clipped off” on the line to the exit side of the jump or gap in order to indicate “the way out”.


Karst — an irregular limestone region with sinkholes, disappearing streams, underground streams, springs and caves.
Karst Window – A cave opening into a cave system or with water flowing across the opening making both upstream and down stream accessible from a single opening.


Lava Tubes — Caves formed (on the surface) by lava flow. Submerged lave tubes like those found in Hawaii where formed when sea levels were much lower.
Light Doubling – System of light communications used in cave diving where dive team follows lead light.
Limestone — a rock that is formed chiefly by accumulation of organic remains (shells or coral), consists mainly of calcium carbonate; frequently contains fossils. The primary rock in most caves.
Line Markers — fall into two classifications:

  • Permanent — Line markers permanently affixed to the line indicating direction and distance to nearest exit.
  • Temporary — Personal markers should be removed after every dive! typically contain information like divers name or initials and sometimes even notches for unmistakable identification.

Line Reels — mechanical devices carried by a cave diver or team of divers to safety retrieve and control the primary guideline while assuring continuous guideline to surface for the entire length of the dive.
Line Trap — An area where a guideline falls or slips where the diving team loses access to it.
Inert Gas — Any gas not metabolized by the body.


M-Value — M stands for maximum. M-Values are maximum allowable tissue tensions. In the early development of decompression theory, it was thought that gas wouldn’t separate appreciably from solution to form bubbles if a specific maximum amount of supersaturation wasn’t exceeded.
Marble — limestone that has been re-crystallized by heat and pressure-are composed of the mineral calcite (CaC03)
Mineral — an inorganic (non-living) substance occurring naturally in the earth and having definite physical and chemical properties.
Mixed Gas – generally an expression or term used to describe helium gas based breathing mixtures.
MOD – Maximum Operating Depth for a breathing mixture, according conventional oxygen toxicity threshold limits – The ACDF adheres to standard 1.6 PO2 at rest / 1.4 P02 for work. MOD = (max PO2 / FO2 – 1) * 10 msw
Mirabillite — hydrous sodium sulfate
Mummified — natural version: a human or animal that has been naturally dehydrated and thereby preserved for a long period of time; humans, bats, and other animals are thought to have been well preserved by the constant humidity and temperature and the presence of salts in the cave soil.


NACD – National Association of Cave Divers.
NDL — No-Decompression Limit. maximum time aloud at depth without requiring stop (s) during final ascent to the surface.
Nitrogen Tension — Nitrogen tension is a calculation of how much nitrogen you take up during a scuba dive. Nitrogen tension is measured by pressure, not volume.
NITROX — any mixture of Oxygen and Nitrogen. for diving purposes we limit our consideration to those mixtures that have a greater percentage of Oxygen than the air we breathe. These mixtures are referred to as oxygen Enriched Air (OEA) Enriched Air Nitrox (EAN) or, sometimes you see the abbreviation, EANx the “x’ being the value or percentage of oxygen in the air mixture. “Example EAN35” would indicate 35% oxygen.

Primary reason for using NITROX:

    • More bottom time (less deco time)
    • Greater safety (within MOD)
    • Less Fatigue

No Decompression Time — The maximum time which can be spent at a given depth such that ascent can be safely made directly to the surface at a prescribed rate.
NSS – National Speleological Society — Organization that believes: that caves have unique scientific, recreational, and scenic values; that these values, once gone, cannot be recovered; and that the responsibility for protecting caves must be assumed by those who study and enjoy them.
NSS-CDS — National Speleological Society-Cave Diving Section — Unlike other certifying agencies the NSS-CDS is an organization open to anyone with an interest in underwater caves. Some of the benefits are bi-monthly issues of Underwater Speleology, a Member’s Manual, discounts on books & materials and voting privileges to decide who should run the organization.


Off Gassing — The elimination of nitrogen (and/or other inert gases) is referred to as “off-gassing” or “de-saturation”. Off gassing begins during final ascent as tension between body’s tissues and the lung’s alveoli equalize.
Oxygen Toxicity — Toxic effect of Oxygen beyond a safe partial pressure. Known to cause convulsions and (in divers) drowning.
Oxyhemoglobin – Hemoglobin that has bonded with inhaled oxygen.


P — Pressure
Paleontology — the study of life from past geologic periods by examining plant and animal fossils.
Paleofeces — fossilized human or animal feces. By studying paleofeces we can learn what humans were eating when exploring a cave thousands of years before.
Passage — in a cave, the corridor created by water and rock falls.
Percolation – Silt dislodged by a divers exhaust bubbles from the cave ceiling.
Permanent Line — Line left in an underwater cave to assist cave divers with underwater navigation. NOTE: A line set for surveying purposes only, is not classified as a permanent line.
PFO — Patent Foramen Ovale, open able flap between atria. See ASD. A shunt between the right and left side of the heart that allows some blood to circulate back through the body without going to the lungs first. This means that micro-bubbles don’t get removed from the blood stream by the lungs efficiently, making DCS more likely. 1in 4 people have a PFO.
pH — a scientific measure of hydrogen ion activity to determine the acid or base level of a substance frequently more intense at the water line.
Phreatic Zone — the portion of the aquifer below the water table.
Phreatite — reddish/ brown/ black crust that forms on the bedrock of a submerged cave composed of mostly iron oxide. bacteria with rough, bubbly-looking (botryoidal) appearance.
Pit — in a cave, a vertical shaft as viewed from above, formed by dripping or falling water through a vertical crack. See dome.
Placement — Positioning of line around an object to control line yet allow for easy removal.
Pneumothorax – Injury where air or another gas enters the pleural cavity. This prevents expansion, and may even cause lung collapse.
P02 – Partial Pressure of Oxygen
Popcorn — a calcite speleothem with the appearance of popcorn.
Pollution — the fouling of water or air with sewage, industrial waste, or other contaminants, making them unfit to support many forms of life. Submerged caves are extremely susceptible to surface leaching of pollutions sources such as nearby dairy farms and petrol filling stations.
Pulmonary Oxygen Toxicity – Damage to the lung and the air passages caused by long exposure to oxygen partial pressures above 0.5.
Primary Reel — Used to get from the open water to the primary line located in the cave. This reel consists of a spool to hold the guideline, a winding knob to retrieve the line, a bolt-snap to secure the reel to the diver when not in use, a lockdown screw to secure the spool from unwinding when line is no longer required, a handle to hold the reel, hub to hold the spool, and a guide to direct the guideline on and off the spool. Normally 80-100 meters of line maximum.
Pulmonary Toxicity — Lorrain Smith (1899) demonstrated, for the first time, that animals breathing oxygen at moderately high tensions over prolonged periods suffered pulmonary damage.


Radon — a heavy radioactive gaseous element formed by the disintegration of uranium and thorium. It is colorless, tasteless, and odorless and found commonly in bedrock and sediments. All rocks and soil, including the limestone found in Mammoth Cave, contain varying amounts of radon.
Recharge Zone — the area from which a body of water is recharged. The cave and groundwater recharge zone may be many miles from the cave itself and may include disappearing springs or “siphons” and sinkholes.
Repetitive Dive — Any dive conducted within a 12-hours period of a previous dive.
Repetitive Group Designation — A letter, which relate directly to the amount of residual nitrogen in a diver a body for 12-hour period following a dive.
Residual Nitrogen (RN) — Nitrogen gas that is still dissolved in a diver’s tissues after he has surfaced.
Residual Nitrogen Time (RNT) — An amount of time, in minutes, which must be added to the bottom time of repetitive dive to compensate for the nitrogen still in solution in a diver’s tissues from a previous dive.
Resurgence — water emitting from a cave supplied almost entirely by the sinking of surface streams as opposed to “exsurgences”.
RIB — rigid hull inflatable boat, also RHIB
Rimbach System — also known as “touch contact” is a technique first developed by Don Rinbach allowing limited communications between team members during low or zero visibility conditions.
Rimstone Dam — thin mineral crusts formed at the edge of some cavern pools as calcite-rich water flows over the edge.
Rule Number One — never dive with people who compromise safety.
Rule of Thirds – gas management regime first conceived by the Sheck Exley: The diver must reserve minimum 2/3’s of the beginning gas supply for the exit trip from the cave.


SCR — Surface Consumption Rate. Also known as SAC (Surface Air Consumption)
SAR Team — Search and Rescue Team
Sacrifice Caves — see ‘show caves’
Safety Spool — Line spool that holds approximately 15 to 30 meters of guideline. that provides the diver with a ready source of guideline to use in the event of lose of guideline.
Safety Stop — A stop made at six meters depth just before surfacing from a dive. Safety stops greatly lower tensions in the very fast tissues because they off gas quickly. Best when performed on pure 100% oxygen.
Saltpeter — a potassium nitrate compound converted from calcium nitrates often found in dry caves and used in making gunpowder.
Sandstone — a sedimentary rock made up of small pieces of rock, usually silicates such as quartz, that have been cemented together over time. Sandstone serves as the caprock on top of the limestone layer Mammoth Cave was formed in.
Saturation — When a tissue has absorbed all the nitrogen it can at any particular depth, it is called saturated of that depth.
Scallops — spoon-shaped hollows dissolved in the cave walls, floors, and ceilings by flowing water; the shape and size of the scallop is inversely proportional to water velocity and direction. .
Sea Caves — Caves formed by wave and weather erosion as opposed to being formed by dissolution.
Sediment — rocks or fragments transported by wind, water, gravity, or ice; precipitated by chemical reactions; or secreted by organisms.
Sedimentary Rocks — a rock formed of fragments transported from their source, usually by water. The sediments are usually laid down in layers. Sandstone and limestone, are both sedimentary.
Selenite — clear gypsum, often found as needle-like crystals in the dirt of undisturbed parts of the cave.
Station – A survey point in a chain of such points in a survey.
Survey – In caving, the measurement of directions and distances between survey points and of cave details from them and the plotting of cave plans and sections from these measurements either graphically or after computation of coordinates.
Shaft — a vertical passage in a cave formed by water dripping or flowing through vertical cracks in the bedrock.
Shale — a sedimentary rock formed by the consolidation of fine particles of clay, mud, or silt. Sometimes known as mudstone or siltstone. This rock along with sandstone forms the confining layer in northern Floridian south Georgian aquifer.
Show Cave — a cave developed for public use, usually with permanent lines installed, could also have novice pull lines, signs, permanent line markers, steal gates installed to keep open water divers out of the cave, , etc. . Also known as commercial caves of sacrifice caves.
SI — Surface Interval (between or after a dive)
Sidemount — specialized equipment configuration whereby tanks are mounted on the divers side just under each arm thereby allowing exploration of areas not normally accessible with back mounted tanks.
Silt — one potential hazard in cave diving. Falls into three classifications:

  • Sand – anything bigger the 1/16 mm.
  • Mud – anything between 1/256 mm and 1/16 mm
  • Clay, decreasing in size and increasing in concern, clay is anything smaller then 1/256 mm and can be measured by settling factor. However some sediments hold electromagnetic charge and will bounce off each other for infinity. Normally caves with “high” flow or current have the least amount of silt.

Silting — disturbing silt can cause total of the visual sense and orientation. Generally the result of careless or improper technique. Stress created from such an event can become manageable through intensive training. Also known as ‘Silt Out
Siltation – same as silting but after the fact.
Single Dive — Any dive conducted after 12 hours of a previous dive.
Single Repetitive Dive — A dive for which the bottom time used to select the decompression schedule is the sum of the residual nitrogen time and the actual bottom time of the dive.
Sinkhole — Sinks involve areas where the collapsing ground reveals an opening into a submerged cave. in a limestone region, often called Karst “windows.” In cave diving can denoted by the debris cone or break down of sediment from ceiling collapse. A sinkhole generally allows access to both the upstream and downstream portions of the cave so extreme judgment must be exercised when calculating gas requirements during initial exploration.
Slip Stream
Siphon – inflowing cave, water flowing into the cave, danger in miscalculating gas requirements.
Soda Straws — speleothems that grow on cave ceilings as thin-walled hollow tubes. They are formed by water flowing inside the tube and depositing rings of calcite around their tips. In the air pocket at Hin Doke cave Krabi these fragile soda straws reach 5 meters!
Soft Mount – no metal to metal connections. Can be cut away with knife.
Soil Zone — the more aerated layers of earth near the surface. Note: The carbon dioxide content in the soil zone can be in excess of 300 times the level found in the outside atmosphere.
Solubility — Some gases are more soluble (capable o being dissolved) than others, and some liquids and substances are better solvents (capable of dissolving another substance) than others. For example, nitrogen is five times more soluble in fat than it is in water.
Solution Caves – those dissolved out of solid rock by acidic waters. Most solution caves are found in carbonate rocks (limestone, dolomite, or marble) or gypsum.
SMB — Surface Marker Buoy (now required for all off shore dives). Also known as DSMB (Delayed Surface Marker Buoy).
Speleology — the scientific study of the cave environment, including the physical, chemical, and biological aspects.
Speleothem — cave formations: secondary mineral deposits formed in caves, caused by the dissolution of minerals (such as calcite) and their subsequent deposition in crystalline form in growing layers in a variety of shapes.
Spelunker — see ‘caver’.
SPG — Submersible Pressure Gauge
Spring — a natural flow of water from the ground, often the source of an above-ground stream.
Spring Run — the surface stream or river created by water discharged by a spring.
Sump – A point in a cave passage when the water meets the roof.
Stalactite — a common speleothem which hangs down from the ceiling.
Stalagmite — a common speleothem which rises up from the caved floor from calcite dripped from the ceiling.
Stewardship —related to the environment, the concept of responsible care-taking; based on the premise that we do not own the resource, but are managers of the resources and are responsible to future generations for their condition.
Suicide Clips — A type of brass clip with a springed gate notorious for creating line traps in caves.
Supersaturation — As pressure drops during ascent after a scuba dive body tissues will temporarily increase to a higher tension than they can contain in equilibrium at the surface or become supersaturated.
Surface Interval — The time which a diver has spent on the surface following a dive; beginning as soon as diver surfaces and ending as soon as he starts his next descent.


Tannic Acid
Temple Cave — sometimes used for meditation or as show caves often lighted with cemented walkway.
Thirds — First proposed by Sheck Exley is a gas management rule based on thirds. A third in, third out and a third of the gas supply reserve in case of emergency. NOTE: rule of thirds is generally considered bare minimum for cave diving.
Tie-Off — A physical wrapping of line around object twice to prevent line movement.
Tower Karst – Cone karst in which the residual hills have very steep to overhanging lower slopes. There may be alluvial plains between the towers and flat-floored depressions within them.
TOX — Slang for central nervous system oxygen toxicity that can happen when diving too deep on air or diving deeper than you should on an enriched air (nitrox) mixture. Also called the Paul Bert Effect, but TOX is easier to remember and lots easier to type.
Travel Mix — Mix such as 50/50 used to travel to or from the bottom. Travel gas is used to get down to the point where depth indicates that you must change to your bottom mix, and then to get up from that point to where you can switch to your deco mixes).
Traverse (in cave diving ) -To enter a cave system from one opening point and exit from another.
Travertine — general term for calcite speleothems.
Trimix — Breathing mix of three gases, typically oxygen, helium, and nitrogen. Composition is quoted as percentage of oxygen / percentage of helium, e.g. 17/60 is a Trimix with 17% oxygen, 60% helium, balance (23%) nitrogen primary advantage over heliox is cost..
Troglobite — an animal that lives its entire life within a cave and is specifically adapted to life in total darkness (some by losing their eyes or lacking pigmentation (color)). Examples include the eyeless fish, shrimp, and crayfish.
Troglophile — animals that can live all their lives either inside or outside a cave. Examples include, salamanders, springtails, and spiders.
Trogloxene — an animal that spends part of its life in caves, but must venture out for food. These animals bring organic materials important to troglophile and troglobite survival into the cave. Examples include crickets, wood-rats, and bats.
Tube – A cave passage of smooth surface and elliptical or nearly circular in cross-section.
Turbidity — Turbidity is caused by stirring up debris on the cave floor or ceiling. It can reduce visibility to zero. Turbidity may be produced by several factors. Sand is heavy and will usually settle to the floor of the cave faster than lighter materials. The general rule is, the lighter the weight of the material producing the turbidity, the slower it is to settle and the longer duration of restricted visibility.
Twilight Zone — the part of a cave near the entrance where light penetrates but does not receive direct sunlight, extending to the zone of absolute dark. An important habitat for many trogloxenes. Also called the “visible light zone”.


Vadose Zone — The zone where voids in the rock are partly filled with air and through which water descends under gravity. The portion of the aquifer above the water table.
Vandalism — the willful or malicious destruction or damage of any public or private property. Includes carving ones initials into the cave wall or ceilings like seen in some popular cave systems in Florida and Mexico.

Vasoconstriction – A decrease in the diameter of blood vessels.

Vauclusian Spring – A spring rising up a deep, steeply-inclined, water-filled passage into a small surface pool.


Watershed — an area of land where all water collects and drains into a common body of water (such as a river or lake).

Water Table —the upper level of the underground reservoir of water; the level below which the ground is saturated.
Weathering — the action of the elements in altering the color, texture, composition, or form of exposed objects, removing material physically or chemically. Water, wind, trees, and chemicals can cause weathering. Wild Cave — a cave in its natural state, not developed for public use, in contrast with show caves.

List of Cave Diving Acronyms

AEDU – Admiralty Experimental Diving Unit
AGE — Arterial Gas Embolism
ANDI – American Nitrox Divers Inc.
ATA – Atmospheres Absolute
CAGE – Cerebral arterial gas embolism
CGA – Compressed Gas Association
CNS – Central Nervous System
C02 – Carbon Dioxide
CPTD – Cumulative Pulmonary Toxic Dose
DCIEM – Defence and Civil Institute of Environmental Medicine (Canada)
DCS – Decompression Sickness
EAD – Equivalent Air Depth
END – Equivalent Narcotic Depth
EAN or
EANx – Enriched Air Nitrox (the x is the percentage of oxygen)
EATD – European Association of Technical Divers
FAQ – Frequently Asked Questions
FFM – full face mask.
F02 – Fraction of Oxygen
FSW – Feet of Sea Water
GPS – Global Positioning System
HPNS – High Pressure Nervous Syndrome
IAND – International Association of Nitrox Divers
IANTD – International Association of Nitrox and Technical Divers
IMHO – In My Humble Opinion
MOD – Maximum Operating Depth
MSW – Metres of Sea Water
N2 – Nitrogen
NACD – National Association of Cave Divers.
NDL – No Decompression Limit
NOAA – National Oceanic and Atmospheric Administration
NSS – National Speleological Society
02 – Oxygen
ONM – Oxygen-Nitrogen Mixture
OTU – Oxygen Tolerance Unit
P – Pressure
P02 – Partial pressure of oxygen
RIB – rigid hull inflatable boat
SAR – Search and Rescue
SCR – Surface Consumption Rate or Semi-Closed Rebreather
SCUBA – Self-Contained Underwater Breathing Apparatus
SI — Surface Interval
SMB — Surface Marker Buoy
SPG – Submersible Pressure Gauge
TDI – Technical Diving International
UK – United Kingdom
UPTD – Units of Pulmonary Toxic Dose