"Third Cave" by Anonymous. This is another one of the "A+ Stories" that I mentioned last week. It's another good one.

Three men and a woman go spelunking. This term refers not to just walking through caves, but getting down and dirty while going places where few humans have ever gone. I have visited caves and have even made love in total darkness in one, but I did not have to rappel or crawl several hundred yards, and so I am not a spelunker. I thought you might want to know that. Anyway, the wife gets separated from the husband and has some great sex, first with one of the other guys and then with both of them. I don't recommend this sort of thing - spelunking, that is - unless you really know what you're doing.

The author does an excellent job of setting up the ambiance, and the action in the second sex scene is especially hot.