Ice cave in the Skaftafell National Park in Iceland. Ice caves collapse and new ones form as each glacier grinds slowly towards the sea. Local guides can help you (without dying) find one. (Visit Skaftafell National Park in Iceland.
Melissani Cave, on Kefalonia Island in Greece, features a sky-blue lake within the cave. The lake is shallow and clear, and is covered with stones at the bottom. The cave was rediscovered in (In Greek mythology, Melissani was the cave of the nymphs.