The Blue Grotto 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 into the cliff at the surface, and is about 150 metres (490 ft) deep, with a sandy bottom.

A sea cave, also known as a littoral cave, is a type of cave formed primarily by the wave action of the sea. The primary process involved is erosion. Sea caves are found throughout the world, actively forming along present coastlines and as relict sea caves on former coastlines. Some of the largest wave-cut caves in the world are found on the coast of Norway, but are now 100 feet or more above present sea level. These would still be classified as littoral caves. By contrast, in places like Thailand's Phang Nga Bay, solutionally formed caves in limestone have been flooded by the rising sea and are now subject to littoral erosion, representing a new phase of their enlargement.