Insanely dangerous, and even well marked, frequently explored caves can abruptly turn deadly even for experienced cavers. Still more terrifying is cave diving, in which explorers descend by up to hundreds of meters through winding, claustrophobic, narrow caves entirely filled with water.

At least with "dry" cave exploration, if you get stuck, you might survive up to several days with a hope of rescue, assuming you can stay hydrated and not freeze to death. With submerged cave exploration, the minute you run into serious trouble, the clock starts ticking with life lasting only as long as your air tanks. The stuff of nightmares.

This YouTube channel covers many of these accidents and incidents really well. https://www.youtube.com/watch?v=M2xs90tbEeY