Dos Ojos Cave, near Tulum, Mexico. Dove a less touristy and more technical part - had been discovered shortly before I dove it and few divers had ever been there. Flashlights were the only light for much of the 2 dives. One minute you're delicately finning through small passages (kicking up the silt can ruin the visibility and make the trip back dangerous), and the next minute you're in an area the size of a cathedral. The stalagmites & stalactites are enormous, prehistoric, and beautiful.