To spelunk is to explore natural caves. Geologists do a lot of spelunking.
This is a weird-looking word, but it has a simple meaning: to explore natural caves. Scientists who study natural formations of the Earth do a lot of spelunking. Through spelunking, you can find evidence of difference minerals, as well as fossils of various animals. Archaeologists spelunk to find remnants of previous civilizations. Some people spelunk as a hobby: they are spelunkers.