Question: Spelunking is the exploration of what, especially as a hobby?
Answer: River cave; Dry cave; Cave; Natural cave; World's deepest cave; Sacred cave; Caves; Fossil cave; Primary caves; Collapse cave; Cave system; Cave systems; Dripstone cave; Cave records; Karst Cave; Primary cave; Caverns; Collapse cavern; Cave complex; Cavern; Natural caves; Cave (natural); Water cave