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Cavern diving is an extension of SCUBA diving where the diver enters a naturally created overhead environment (so that a direct vertical ascent to the surface is not possible). If the diver were to continue far enough into this environment, it would be considered Cave diving (also see Penetration diving); however, a cavern dive is limited to the outermost zone of this environment (the cavern zone), with barriers which are not universally agreed upon.

Cavern diving is most common around the entrances to underwater Caves. It is taught primarily to introduce recreational sport SCUBA divers to diving in overhead environments, but not to directly teach cave diving.

Each training agency defines the cavern zone slightly differently, though all agencies define it with the same types of measurements:

• Daylight zone: a cavern diver must stay within the area directly illuminated by daylight.
• Limited penetration: a cavern dive is always limited by how far from the surface the diver is (the sum of the depth plus the distance traveled into the cavern).
• Limited depth: training agencies limit the maximum depth of a cavern dive, but this number is highly dependent on the agency.
• Minimum visibility: cavern dives require that the water have a certain amount of clarity. Again, the amount varies from agency to agency.
• No restrictions: cavern divers may not enter passages too small for two divers to exit side-by-side. Swimming side-by-side may be required in an emergency if the divers are forced to share air during the exit; cave divers use more advanced equipment to solve this problem.

In the past, there were many cave diving fatalities involving divers who had no formal training in cave diving. Cavern diving courses were developed to introduce interested divers in some of the techniques of cave diving so they could apply them to cavern dives, while also imparting on them the importance of seeking additional training to undertake cave diving. The largest difference between a cavern diving class and a cave diving class is the equipment — while cave divers typically have highly specialized equipment, a diver can take a cavern class with normal open water SCUBA gear. The only special equipment that most agencies require is two lights per diver and one reel per dive team.

The rest of the course focuses on equipment modification (adjusting equipment to optimize it for cavern diving) and some knowledge and techniques borrowed from cave diving.

For those interested in learning how to cave dive, cavern diving also serves as an introductory course. Certified cavern divers may progress to the next levels of cave training.

Almost all major SCUBA training agencies offer a cavern class, though only certain instructors in each agency have the training required to teach it. These classes are adapted from the standards set by the cave diving training agencies, which offer their own cavern classes as well. For a list of both, see SCUBA diving and Cave diving. These courses usually take two to three days to complete.