Caverns, caves and tunnels: not for claustrophobics
Underwater caverns, caves and tunnels come in many varieties – from freshwater springs to volcanic island walls and limestone coasts, to Antarctic icebergs and coral reefs. Many of these locations offer scuba divers enclosed spaces with a mysterious although potentially dangerous attraction. Deeper caves are not for the weak hearted divers, and certainly not for the claustrophobics. It depends of course on the amount of confinement: the shallower caverns in general have sufficient orientation points, often with patches of blue at the entrance or end, or light beams breaking through crevices and holes in the ceilings. But penetration in the interior mazes of deeper tunnels and caves is a different story: more for the brave, well trained adventure and risk seekers.
Types of caves Seawater caves come in two varieties, littoral caves and submerged land caves. Littoral caves are created by erosion: the constant action of waves attacks the weaker portions of rocks lining the shores of oceans. Many of these can be found along the coastline of Mediterranean and its smaller islands. Related to these littoral caves are the tunnels and caverns found in coral reef formations in the open sea, where movements of the sea have caused openings, passages and crevices in the more fragile parts of limestone beds or dead coral. An example are the caverns of Fury Shoals and Shab Claudia in the southern Red Sea. These caverns and swim-throughs have become a hot spot for UW photographers eager to capture the light beams breaking through cracks and holes of the ceiling of the caverns (see picture on the frontpage).
Submerged land caves Other underwater caves were originally land caves that became partially or completely submerged through rising sea levels over thousands of years. Most of these caves are formed in limestone rock where calcite (calcium carbonate) is the main mineral. Here nature’s scenario followed five big steps that reflect the coming and go of successive periods of climate change: 1) acid rainwater dissolves the limestone, carves caves and tunnels deeper below and subsequently fills the network of caves and passages 2) water drains out of the cave through lowering sea level and/or tectonic lifts, 3) stalactites and stalacmites are formed in the now dry caves by leaking rainwater, and 5) rising sea levels through melting ice submerge the caves again with salt water. Caves with ceilings higher than sea level were partially flooded, leaving some remaining air space.
At some locations underground river systems entered the caves and tunnels with sweet water, mixing with high density lower salt water layers. An example are the Cenotes of the Yucatan Peninsula where the spectacular underground rivers formed a vast labyrinth of passageways and tunnels, flooded with crystal clear water. Some of the partially air filled cave ceilings became too thin to hold their own weight, collapsing and creating sinkholes or natural windows as entrances to the underground river system. In these sink holes divers can experience the halocline, the boundary where the shallower fresh water on the top mixes with the sea water below resulting in amazing mirror and light effects. It is said the Cenotes were once used by the Maya Indians for sacrificial offerings. Recently the worlds-longest-underwater-cave-system of 347km (216-mile) was identified at the Yucatan peninsula. This connects the Sac Actum cave with the Dos Ojos tunnel complex. Probably this once was the Maya's metaphorical underwater world to which the cenotes formed the entry.
In the Yucatan cave systems the sweet water of the underground rivers mixes with the salt water layers. But other caves, for example in Florida, contain only sweet water produced by underwater rivers. Florida’s best diving caves are hidden in the northwestern corner of the state in Ginnie springs. This complex of caves and tunnels is fed solely by freshwater springs that flow through mazes of limestone passageways. The springs have the reputation to offer strange sights of underwater chambers—fossils, stalactites and sunlight beaming in from holes in the cave ceilings.
Grand Bahama has the second largest underwater cave system in the world, with over 32,000 feet of mapped tunnels (see Bens cave). The caves were formed during the last Ice Age when the sea level was much lower, leaving most of the Bahama banks that are now covered in water high and dry. In the subsequent meltdown the sea level rose again and the caves were reclaimed by the sea. Another notable example of a submerged saltwater cave is the Cosquer Cave located in the south of France near the Calanques of Marseille. The entrance to the cave is located 37 m (121 ft) underwater, due to the Holocene sea level rise. The cave contains some miraculous prehistoric rock art engravings. It was discovered in 1985 by and named after diver Henri Cosquer, but its existence was not made public until 1991, when three divers became lost in the cave and died. During the glacial periods of the Pleistocene, the shore of the Mediterranean sea was situated several kilometers to the South and the sea level up to 100 m (330 ft) below the cave entry.
Hazards, apparatus and training Good buoyancy control, trim and finning technique help to preserve visibility in areas with silt deposits. This counts for all caves including the more friendly and shallower caverns. As said, penetrating in the deeper caves and tunnels is more for technical divers trained in procedures to survive in the dark and dangerous submerged labyrinths. Often a guide line (permanent or temporary) between the dive team and outside of the flooded cave ensures that divers follow the correct routes. Regular cave divers carry redundant equipment: for almost every piece of equipment they carry a spare. This is to make sure that if something undergoes failure, there's a replacement to take over and allow a safe return to the surface. It could include extra lights, an extra mask, regulator, safety line, or a piece of equipment that ensures a diver's survival, like an oxygen tank. Many cave divers use side mounted cylinders that facilitate switching tanks and crawl through narrow passages. Rebreathers are sometimes used for longer bottom times and to avoid bubbles. Cave divers are also trained to control situations like getting out of air, avoiding stress and panic, navigating with zero visibility, swimming back to the entrance of the cave. Helping and freeing a buddy that gets entangled in a safety line etc.
The friendly caverns More shallow and divers friendly caverns have become a ’special’ for UW photographers*. They require shooting with natural ambient light with your ISO cranked up (say to 6400), large apertures and possibly also lower shutter speeds. Here UW photographers try to capture the light beams breaking through gaps in the ceiling, preferably cutting the interior in diagonal angles like cathedral lights. A diver on the background, perhaps aiming a torch could be an interesting subject to fill up the scene. The major concern in caverns are divers getting too close to the floor kicking up silt, which means the end of visibility. A snooted strobe can be fine to create a special lighting effect, for example to light up a colorful branch of soft coral at the entrance of the cavern, without affecting its background.
Sources and links:
The Cave Beneath the Sea: Paleolithic Images at Cosquer by Jean Clottes and Jean Courtin (1996) Harry N. Abrams, Inc., New York ISBN 0-8109-4033-7 English translation by Marilyn Garner from the French edition
*Underwater Photography Masterclass. Alex Mustard. Ammonite Press, 2016, chapter Chapter six.