Blue Grotto

The Blue Grotto (Italian: Grotta Azzurra) is a sea cave on the coast of the island of Capri, southern Italy. Sunlight shining through an underwater cavity is reflected back upward through the seawater below the cavern, giving the water a blue glow that illuminates the cavern. The cave extends some 50 metres (160 ft) into the cliff at the surface, and is approximately 150 metres (490 ft) deep, with a sandy bottom.

Cave data from Wikidata (CC0). Summary from Wikipedia (CC BY-SA).