"Spelunk" has to be one of my favorite ridiculous-sounding words! It has a simple definition: to explore caves.
To be a spelunker, though, requires a bit of work. You must know where the caves are and how to do your exploring in a safe and environmentally responsible way, but, the rewards can be great. Here's an article from the British newspaper, the Daily Mail, on a crystal cave found in Iceland. I think you'll agree, the photos are stunning!
To learn more about caves and the in and outs of spelunking, look for one of these:
Brimner, Larry Dane. Caving: Exploring Limestone Caves. [J 796.52 BRI]
Lindop, Laurie. Cave Sleuths. [YA 551.447 LIN]
Tabor, James M. Blind Descent: The Quest to Discover the Deepest Place on Earth. [796.525 TAB]
Zappa, Marcia. Caves. [J 551.447 ZAP]