Grotto dello Smeraldo in Conca dei Marini, Italy

This cave, filled partially with water, has grand arching ceilings reminiscent of a cathedral. An opening under the water allows sunlight to come up from below, illuminating the water and revealing the beautiful turquoise color you see throughout the Amalfi Coast. The effect is haunting and lovely.

Buy a ticket to Grotto dello Smeraldo in Amalfi, then take a boat from the Amalfi Harbor and disembark at the grotto shore; a guide in a large rowboat will give you a tour of the cave, pointing out interesting rock formations and, at the end, a small nativity scene that has been placed beneath the water by divers. The nativity is lit artificially from above, and looks quite creepy through the green water.