Having sexual intercourse with a girl who has a very wide and stretched out vagina. The vagina is very cave-like due to its constant past use. If you are considered to be spelunking, then the girl is a whore, and you will most likely contract a sexually transmitted disease.
When your at a party talking toyour friends and a hot chick walks over that you want to have sex with. Use the term spelunking to make it seem like your not a perve. Just say hey man I want to go spelunking