Cave diving is underwater diving in water-filled caves. 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. Despite the risks, water-filled caves attract scuba divers, cavers, and speleologists due to their often unexplored nature, and present divers with a technical diving challenge.

Cave diving venues can be found on all continents except Antarctica, where the average temperature is too low for water to remain liquid in caves.

There are few flooded caves in Africa which are known and accessible. There are several in South Africa, a few in Namibia and Zimbabwe, and some large caves recently discovered in Madagascar.

There are a large number of flooded caves in the limestone regions and other regions of Asia, particularly in the karst regions of China and Southeast Asia. Some are accessible for recreational cave diving, but most have probably not yet been found or explored.

Australia has many spectacular water filled caves and sinkholes, many of them in the Mount Gambier region of South Australia.

Europe has a large number of flooded caves, particularly in the karst regions.

North America has many cave diving venues, particularly in Florida, US, and the Yucatan Peninsula of Mexico.

South America has some cave diving venues in Brazil.

Africa There are few flooded caves in Africa which are known and accessible.

Madagascar Anjanamba Cave is Africa's longest known flooded cave, with over of passages.

South Africa Boesmansgat is a sinkhole in the Northern Cape province has been dived to depth. Wondergat (more of a cavern, but with a small cave section) Komati Springs – a flooded mine Cango Caves have flooded sections but are not open to the public in those areas.

Zimbabwe

Asia There are a large number of flooded caves in the limestone regions and other regions of Asia. Some are accessible for recreational cave diving, but most have probably not yet been found or explored.

China The Karst landforms in China's Guangxi, Yunnan, Guizhou and other provinces are extremely conducive to the development of caves, and thus give rise to rich groundwater features. 10 underground rivers with a length of more than 80 kilometers have been discovered, with hundreds of caves, skylights, "Feilai Lake" and other landscapes. However, in Guangxi and other places, the caves available for diving also have the characteristics of large water flow, fast speed, deep caverns, complex structure, and harsher cavern environment.

In recent years, Nanning Red Army hole, the sun and the moon hole, Liuzhou Fairy Cave, a series of cave systems attracts a large number of cave diving enthusiasts.

Philippines Pawod Underwater Cave in Mactan Island, off the island of Cebu in the Central Philippines was discovered in 2001 by Dr. Alfonso Amores. The discovery opened cave diving for the locals who erstwhile could not afford the expensive course in Florida. In 2012, Jake Miranda, Dr. Alfonso Amores and Bernil H. Gastardo founded the Filipino Cave Divers (FCD). Exploration opened the freshwater caves of Casili, Balamban in Cebu Island and Enchanted River Underwater Cave in Hinatuan, Surigao del Sur in the big southern island of Mindanao. Several more caves in Mindanao have since been discovered and explored – Bababu Lake (Dinagat Island), Pamutuanan Cave (Lianga, Surigao del Sur) and Campomento Cave (Lanuza, Surigao del Sur).

West Timor, Indonesia

There are a number of freshwater filled cave sites located near and within Kupang, the principal city in West Timor and the provincial capital of East Nusa Tenggara. At least 2 sites have been described: Gua Oehani (translates as the Oehani Cave) and Gua Kristal (translates as the Crystal Cave).

As of 1999, there was no requirement to hold cave diving certification to dive either of these sites. However, competence equivalent to CDAA Advanced Cave and Cave/Advanced Cave respectively appears to be appropriate for these sites.

Australia

Australia has many spectacular water filled caves and sinkholes, but unlike the UK, most Australian cave divers come from a general ocean-diving background.

The "air-clear" water conditions experienced in the sinkholes and caves found in the Lower South East (now called the Limestone Coast) of South Australia (SA) has attracted many visiting divers with the first cave and sinkhole dives taking place in the very late 1950s. Until the mid-1980s divers generally used single diving cylinders and homemade torches, and reels, resulting in most of their explorations being limited. Mixed-gas and rebreather technologies can now be used in many sites. The area is usually known within the cave diving community as the Mount Gambier region.

A series of incidents between 1969 and 1974 in the former Lower South East of SA in which 11 divers died (including a triple and a quadruple fatality) in the following four karst features – Kilsbys Hole, Piccaninnie Ponds, Death Cave (also known as Alleyns Cave) and The Shaft – created much public comment and led to the formation of the Cave Divers Association of Australia Inc. (CDAA) in September 1973. The introduction of a testing program by the CDAA in 1974, which involved the assessment of prospective cave divers' cave diving ability led to a reduced fatality rate. In 1989, this testing system was replaced by a training system which consists of three levels of qualification – Deep Cavern, Cave and Advanced Cave. Five further deaths have occurred since 1974; two died at Piccaninnie Ponds in 1984, one person died at Kilsbys Hole in 2010, and two people died in separate incidents at Tank Cave in 2011 including noted cave diver Agnes Milowka.

During the 1980s, the Nullarbor Plain was recognized as a major cave-diving area, with one cave, Cocklebiddy, being explored for more than 6 kilometres, involving the use of large sleds to which were attached numerous diving cylinders and other paraphernalia, and which were then laboriously pushed through the cave by the divers. In more recent years divers have been utilizing compact diver-towing powered scooters, but the dive is still technically extremely challenging.

A number of other very significant caves have also been discovered during the past 20 years or so; the 10+ (Lineal) kilometre long Tank Cave near Millicent in the Limestone Coast, other very large features on the Nullarbor and the adjacent Roe Plain as well as a number of specific sites elsewhere, and nowadays the cave diving community utilizes many techniques, equipment and standards common internationally.

The Cave Divers Association of Australia is the major cave diving organisation in Australia and is responsible for the administration of cave diving at many sites. All cave diving in the Limestone Coast as well as at some New South Wales sites and the Nullarbor requires divers to be members of the CDAA, whether in the capacity of a visitor or a trained and assessed member. A number of other organisations participate in cave diving activities within Australia. The Australian Speleological Federation Cave Diving Group which was formed in 2005 coordinates projects focused on exploration and mapping at sites throughout Australia. The following diving training organisations offer courses in various aspects of cave diving via instructors either resident in Australia or visiting from overseas – Global Underwater Explorers, International Association of Nitrox and Technical Divers and Technical Diving International.

New Zealand

Europe

Bosnia and Herzegovina Una National Park, in the Bihac municipality, encompasses the valley of the Una River and is a hotspot for speleological research expeditions. Thanks to the area's position between the end of two large karst systems: Plješivica and Grmeč, the river abounds with shallow water sources and a significant number of underwater caves. So far, 10 large caves have been registered, only five of which have been partially examined. Divers who have explored Klokot Spring have managed to reach a depth of , though the bottom of the spring has yet to be touched. Another cave has been explored to the length of , and is said to be an ideal destination for cave divers in training.

Croatia

Czech Republic Hranice Abyss, or Hranická Propast, is the world's deepest known underwater cavity. It beat the previous record holder, Pozzo del Merro in Italy, by . Polish explorer Krzysztof Starnawski, who has explored the upper parts of the flooded fissure for decades, has determined the depth to be at least . As of 2017, divers have yet to reach the bottom of the cave.

France

Germany

Hungary Molnár János Cave is an active thermal water cave located on the western banks of the Danube river. Belonging to the Buda Hills karst system, which is home to many of Budapest's famed hot springs, the temperature of the water sits between year-round. Whilst the first underwater explorations began in the 1950s, the cave wasn't successfully charted until the 70s and 80s when a group of divers mapped out more than of the cave system. The cave, which contains a labyrinth passages, currently stretches to over though there are many more untouched sections to explore.

After the death of a diver in 2011, the Hungarian government imposed strict rules for diving. To dive at Molnár János, divers must have at least an intro-to-cave-diving certification or they cannot dive in the tunnels; they must have dive insurance that covers cave diving; and they must have air supplied in the form of double tanks, stages or a rebreather. In addition, each dive is accompanied by a guide, to minimize the risk of a wrong turn.

Iceland Silfra is a fissure between the North American and Eurasian tectonic plates in Thingvellir National Park. It is the only place in the world where you can dive or snorkel directly in a crack between two tectonic plates. The tunnel from the entrance of Silfra to the cave is commonly known as ‘The Tunnel’, due to the strong current that ‘flushes’ divers attempting to swim through. This tunnel is rarely dived due to the dangerous head-first vertical dive needed to pass through. The deepest part of the cave is at least deep, and has only been dived by a few people because of the narrow and unstable passages.

Ireland Pollatoomary in the Partry Mountains near Killavally, Westport, County Mayo, is the deepest explored underwater cave in Ireland.

Italy

In the north west of Sardinia, close to Porto Conte bay, Alghero territory, there are more than 300 caves above and below water, with about 30 large, and many smaller, underwater sea caves in the limestone cliffs of Capo Caccia and Punta Giglio. The Nereo Cave is the most important and is considered the largest in the Mediterranean Sea. On the east side of Sardinia there are many underwater cave systems starting from the Gennargentu Mountains, with underwater rivers. There is a fresh water cave of more than depth in this area.