The Blue Grotto is a spectacular sea cave on which Italian island?

The Blue Grotto (Italian: Grotta Azzurra) is a sea cave on the coast of the island of Capri, southern Italy. Sunlight, passing through an underwater cavity and shining through the seawater, creates a blue reflection 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.

The cave is 60 metres long and 25 metres wide. The cave mouth is two metres wide and roughly one metre high. For this reason, entrance into the grotto can only be achieved when tides are low and the sea is calm. Without calm seas and low tides, the grotto becomes inaccessible, as the 1-metre entrance is impossible to pass. To enter the grotto, visitors must lie flat on the bottom of a small four-person rowboat. The oarsman then uses a metal chain attached to the cave walls to guide the boat inside the grotto. Swimming in the grotto is forbidden, both for safety reasons and to preserve water clarity.