There was a really amazing bbc documentary about a year ago about the yucatan and divers who are trying to map the network of underground caves there. It was one of the few things I've seen recently that really stirred my imagination, but, yes, extreme. Can't remember the name of the pits they were diving in ("xxxxxx pit"), but one of the things that stands out was an especially deep, empty, lifeless one where the divers eventually reached an acidic layer of rotting plant life, and also the way the water is in layers - clear above cloudy or something? That was a bit of personal nostalgia (about a documentary!!!?)