Observations from Jonathan Volanthe, "one of the British cave divers who helped rescue 12 schoolboys from a flooded cave in Thailand in 2018 " [who] says cave diving carries risks fundamentally different from open-water diving"

"If something goes wrong, you can't simply head to the surface because there's usually something that's preventing that ... Quite often in caves as well, it's very easy to swim in somewhere and then find you stirred some silt up," Volanthen says.

The combination of not being able to ascend to the surface and a poor-visibility exit makes it "much more difficult to get out" if a diver hits trouble, he says.

Adding depth to the equation, he says, exacerbates the dangers.

"The deeper you are, the more air you use, or the more gas, depending on what you're breathing ... Depth generally equals an increase in danger," Volanthen says. Divers must carefully manage gas supplies to ensure a slow ascent, to reduce the risk of decompression sickness.

"Pressure [as a result of depth] creates a situation where gas dissolves into the bloodstream, and that means you have to ascend slowly," he says.