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

The death of John Jones in Nutty Putty cave is the stuff of nightmares.

He was trapped upside down for 27 hours before dying.

https://science.howstuffworks.com/environmental/earth/geolog...

We used to joke that our student caving club was really very safe, having only had seven fatalities over the 50 years it had been operating.

In reality we had a lot of expertise and took safety very seriously. The club had always done a lot of exploration of new caves and other intrinsically tricky activities, and six of the seven deaths were in the same accident (the Mossdale Caverns tragedy of 1967). Our parties of freshers were in very limited danger being trained in well-known and well-explored caves. But still.

There's not much to find for 99.99% of both of those. Like, exploring a distant planet versus exploring actual vacuum "space", is similar to exploring an undersea thermal vent versus exploring millions of square miles of featureless silt around it.