Socotra is a little-visited mythical island with stunning beaches and the longest known cave in Africa, at 21 kilometers in length. I don't know if this cave is fully mapped or explored, or if the island has been fully explored for caves. Either way, this island appears to be a paradise for the caver and the traveler, and I hope to get here soon once Yemen becomes a little more stable. Mainland Yemen as well likely has some magnificent caves to be discovered, but that's for another time.