This word may seem to have German written all over it but it's actually Latinate: from spelunk, meaning "cave." The word designates an explorer of caves and is used mostly in American English, in preference to the more technical and refined speleologist. You can describe what a spelunker does as spelunking, or if you want to make sure you're understood, you can call it caving, or in the United Kingdom and Ireland, potholing.