Cave diving

Cave-diving is underwater diving in water-filled caves. It may be done as an extreme sport, a way of exploring flooded caves for scientific investigation, or for the search for and recovery of divers or, as in the 2018 Thai cave rescue, other cave users. The equipment used varies depending on the circumstances, and ranges from breath hold to surface supplied, but almost all cave-diving is done using scuba equipment, often in specialised configurations with redundancies such as sidemount or backmounted twinset. Recreational cave-diving is generally considered to be a type of technical diving due to the lack of a free surface during large parts of the dive, and often involves planned decompression stops. A distinction is made by recreational diver training agencies between cave-diving and cavern-diving, where cavern diving is deemed to be diving in those parts of a cave where the exit to open water can be seen by natural light. An arbitrary distance limit to the open water surface may also be specified.
Equipment, procedures, and the requisite skills have been developed to reduce the risk of becoming lost in a flooded cave, and consequently drowning when the breathing gas supply runs out. The equipment aspect largely involves the provision of an adequate breathing gas supply to cover reasonably foreseeable contingencies, redundant dive lights and other safety critical equipment, and the use of a continuous guideline leading the divers back out of the overhead environment. The skills and procedures include effective management of the equipment, and procedures to recover from foreseeable contingencies and emergencies, both by individual divers, and by the teams that dive together.
In the United Kingdom, cave-diving developed from the locally more common activity of caving. Its origins in the United States are more closely associated with recreational scuba diving. Compared to caving and scuba diving, there are relatively few practitioners of cave-diving. This is due in part to the specialized equipment and skill sets required, and in part because of the high potential risks due to the specific environment.
Despite these risks, water-filled caves attract scuba divers, cavers, and speleologists due to their often unexplored nature, and present divers with a technical diving challenge. Underwater caves have a wide range of physical features, and can contain fauna not found elsewhere. Several organisations dedicated to cave diving safety and exploration exist, and several agencies provide specialised training in the skills and procedures considered necessary for acceptable safety.

Environments

Two types of overhead diving environment are defined in recreational cave diving: Both a cave or a cavern are natural voids under the Earth's surface.
The formation and development of caves, known as speleogenesis, can occur over the course of millions of years. Caves can vary considerably in size, and are formed by various geological processes. These may involve a combination of chemical processes, erosion by water, tectonic forces, microorganisms, pressure, and atmospheric influences.
Most caves are formed in limestone by dissolution. Caves can be classified in various ways, including a contrast between active and relict, where active caves have water flowing through them and relict caves do not, though water may be retained in them. Types of active caves include inflow caves, outflow caves, and through caves. In speleology, a cavern is an enlarged space within a cave—but in cave diving different terms are commonly used, as 'cavern' implies visual proximity to an exit.

Cave

The underwater cave environment includes those parts of caves which may be explored underwater. Recreational cave diving can be defined as diving underground beyond the reach of natural daylight, as a way of distinguishing between cave and cavern diving. In this context, while artificially formed underground spaces such as mines are not generally called caves by divers, the activity of diving in them is often classed as cave diving for training and certification purposes by diver training agencies.

Cavern

Cavern diving is an arbitrarily defined, limited scope activity of diving in the naturally illuminated part of underwater caves, where the risk of getting lost is small, as the exit can be seen, and the equipment needed is reduced due to the limited distance to surface air. It is defined as a recreational diving activity as opposed to a technical diving activity on the grounds of low risk and basic equipment requirements.

Procedures

The procedures of cave-diving have much in common with procedures used for other types of penetration diving. They differ from open-water diving procedures mainly in the emphasis on navigation, gas management, operating in confined spaces, and that the diver is physically constrained from direct ascent to the surface during much of the dive.
As most cave-diving is done in an environment where there is no free surface with breathable air allowing an above-water exit, it is critically important to be able to find the way out before the breathing gas runs out. This is ensured by the use of a continuous guideline between the dive team and a point outside of the flooded part of the cave, and diligent planning and monitoring of gas supplies. Two basic types of guideline are used: permanent lines, and temporary lines. Permanent lines may include a main line starting near the entrance/exit, and side lines or branch lines, and are marked to indicate the direction along the line to the nearest exit. Temporary lines include exploration lines and jump lines.
Decompression procedures may take into account that the cave diver usually follows a very rigidly constrained and precisely defined route, both into and out of the cave, and can reasonably expect to find any equipment such as drop cylinders temporarily stored along the guideline while making the exit. In some caves, changes of depth of the cave along the dive route will constrain decompression depths, and gas mixtures and decompression schedules can be tailored to take this into account.

Skills

Most open-water diving skills apply to cave-diving, and there are additional skills specific to the environment, and to the chosen equipment configuration.

Good buoyancy control, trim and finning technique help preserve visibility in areas with silt deposits. The ability to reverse kick to back out of restrictions where there is no space to turn around is useful.
*Finning skills: Frog kick, which avoids up- and downward directed vortices and are less likely to disturb silt on the bottom or loose material on the ceiling, and modified frog kick, a version which is more suited to narrow spaces; modified flutter kick, a version of flutter kick which minimises downward directed vortices; back kick, which produces thrust towards the feet, used to move backwards along the long axis of the diver, and helicopter turns, which rotate the diver on the spot around a vertical axis, using lower leg and ankle movements.
The ability to navigate in total darkness using the guideline to find the way out is a safety critical emergency skill. Line management skills required for cave-diving include laying and recovering guide lines using a reel, tie-offs, the use of a jump line to cross gaps or find a lost guide line in silted out conditions, identifying the direction along the guideline leading to the exit, and the skills of dealing with a break in a guideline.
Emergency skills for dealing with gas supply problems are complicated by the possibility of the emergency occurring in a confined space and low visibility or darkness, and at a considerable horizontal distance from a free surface to the atmosphere.
Communicating by touch and light signals.
Providing and receiving emergency breathing gas while swimming through narrow spaces.
Line management

The essential cave-diving procedure is navigation using a guide line. This includes laying and marking line, following line and interpreting line markers, avoiding entanglement, recovering from entanglement, maintaining and repairing line, finding lost line, jumping gaps, and recovering line, any of which may need to be done in zero visibility, total darkness, tight confined spaces or a combination of these conditions.

Laying a cave line: the procedure of running line to avoid snagging on the diver and so that it runs fairly straight between s, placing the line so it can all be seen and reached, so it can be followed in good or bad visibility, avoiding s, and securing the line sufficiently at suitable places to keep it in position.
*Making placements - securing the guideline as it is being run and the choice of primary and secondary tie offs.
*Temporary line – line that is laid on the way into parts of a cave without permanent line, and recovered to the reel on the way out.
*Permanent line – line that is thicker, and therefore easier to see, stronger, and more abrasion resistant, and more securely fastened, intended to be left in place for use by other divers. It may be secured to placements at closer intervals to facilitate finding the other end and reconnection in case of a break. It is not recovered during exit, and will generally be marked.
** is heavy duty permanent guide line of a bright yellow colour and kernmantle construction that leads to the exit. The colour was chosen to make the line more easily identifiable while remaining highly visible, and a heavier duty line also helps in identification by feel and makes the line more durable. In some regions the consensus among cave diving professionals is to reserve gold line for cavern areas which are used for tours. In some cases the gold line extends to the naturally illuminated zone, while in others it will start inside the dark zone, to avoid tempting incompetent and inadequately equipped divers from following the line into the cave.
Marking line: Permanent line is marked to indicate the direction to the nearest exit, and to indicate where dive teams have passed but have not yet returned by looping the line onto a line marker, ensuring that a directional marker points in the correct direction, and that all markers are sufficiently securely attached, while remaining easily removable if temporary.
*Directional markers - cave arrows used to indicate the way out along the line.
*Personal markers – cookies – temporary markers to indicate that a diver has passed beyond a point but has not returned yet, particularly where the group leaves the permanent line or makes a jump to a secondary permanent line. A personal directional marker may be attached to a line to indicate that the owner has gone in that direction, which can happen after divers are separated if one finds the line but not the other divers, and decides to exit independently.
Following a line – the skills of navigating back out of a cave using the line as a guide, particularly in the dark and low visibility.
Finding a lost line: although cave-diving procedures are intended to minimize the risk of losing the guide line, it can happen, and as the chance of finding one's way out without the line are drastically reduced, losing the line is considered a life-threatening situation, and the diver must be competent in the methods for relocating the line in all reasonably foreseeable circumstances, which include a bad silt-out, total darkness, and loss of contact with the other divers in the team. Methods of search should prevent the diver from drifting away from the vicinity of the line, so the start position for the search should be secured by tying off a search line. In principle, if the diver feels their way around the cross section of the cave orthogonal to the line direction until they get back to the starting point, the guideline should be either found directly, or be within the loop of the search line, but even this is not guaranteed to work, as the shape of the cave and limited breathing gas supplies could make this impossible.
Fixing a line break, This requires the ability to tie reliable knots, which may have to be done in bad visibility. It also requires the diver to find the other end of the break. On the way into a cave a line break is an inconvenience, as if the other side cannot be found, the divers can still find their way out. On the way out a line break is a life-threatening emergency until the other end is found, as gas supply is limited and there is no infallible way to find the other end while there is enough gas to complete the exit.
Jumping a gap to a branch line – this involves tying off to the permanent line in a way that is unlikely to come loose, but may be released quickly on the return. Passing a loop on the end of the jump line round the permanent line and over the reel is a standard method. A tag on the end of the loop helps release it quickly. This method cannot be inadvertently released by other divers following the main line.
Recovering temporary line – this is done by the last diver in line on the way out, so the others can follow the line out of the cave, even in reduced visibility, without risking losing the line. They may prepare the line for the reel operator, by releasing tie-offs, which cuts down on delays. In an emergency exit the reel and line may be temporarily abandoned, as it can be retrieved later. The reel operator keeps a light tension on the line and tries to keep it evenly distributed over the width of the reel while winding in. The divers further ahead on the line can feel the presence of others behind them by line tension and small movements of the line as it is reeled in.