Marine

Most of those who have died in these flooded caves were not cave-divers but open-water divers whose curiosity got the the better of them.

Not understanding caves, caving or cave-diving, they did not appreciate the specific features, problems, hazards and techniques inherent to caves and cave-diving; so were unable to avoid running into difficulty, and then extricating themselves when they did.

You'd have to read the relevant cave-rescue reports to find out what happened in each incident (as far as the rescuers could determine). However I can offer some generic reasons:

* Some may not have known the real and common risk of no-visibility for the return, thanks to stirring up silt with their fins and exhaust-bubbles on the way in; nor the use of a line-reel to guide you back through the underwater "fog". It gives the effect of swimming in coffee-grounds, blocks your view ahead and is extremely disorientating.

* Or if they did know, and tried to use a line-reel, they may have not know how to lay it properly past obstructions such as low sections. Or they lost their line-reel but didn't know how, or weren't equipped, to find it again.

* Other may have broken, or never knew, the "Thirds Rule" of air supply. And under stress, your breathing-rate and intensity increase.

* Some may have lost bouyancy-control and ended up trapped against the roof, perhaps in a small dome or between phreatic pendants (if you don't know what those are don't throw yourself into a sump) and been unable to sort themselves out.

* Depth-associated. I don't know if "The Bends" is a problem underwater or only after surfacing but some of these resurgence caves are very deep, with vertical entrances (flooded shafts) into horizontal passages maintaining that depth for long distances, hence pressure-exposure times. Caves of that nature impose very careful gas- and dive- planning. Some lost divers might have been disorientated by nitrogen-narcosis - an intoxication that blunts your reactions and ability to think straight.

* Equipment failure. That can happen! No duplicated kit? You can't depend on buddy techniques in these situations, or they might have to recover your mate's body as well as your own. Very experienced cave-divers would help each other to a point, but only to a point. At least in the sea you can both surface, hopefully without both incurring decompression sickness.

Experienced Cave-divers have died in sumps such as the Blue Holes and elsewhere, as even with all their training, experience and fitness things can go unexpectedly wrong, including physiological failures such as heart-attacks.

Ironically, I know of all these hazards and difficulties, but though a caver, my own participation in cave-diving is acting occasionally as "sherpa". I am not a diver - cave or open-water!

Commercial dive-centres have latched onto resurgence diving as some sort of so-called "extreme sport" on its own, devoid of any connection to caving, which is not an "extreme sport", save only that a sump has a roof of solid rock instead of open air. Some advertise here on Answers. I fear as a result, more of the sort of accident asked in the question. Indeed, only a few years ago there was just such a fatality in the Pleurdal Rising, in Norway - a very deep sump. His body was recovered by a combined Norwegian / French / British team of very highly skilled cave-divers.