The Blue Grotto is a sea cave located on the coast of the island of Capri, in southern Italy, famous for its otherworldly blue glowing waters. The grotto's fluorescent glow comes from a large underwater cave opening beneath the entrance used by visiting boats that illuminates the grotto's water from below, like neon lights in a pool. Known since the days of the ancient Romans, the Blue Grotto’s intense and brilliant blue has been fascinating visitors ever since.

The Blue Grotto is 60 meters long and 25 meters wide. The clear blue waters go down 150 meters until it reaches the sandy bottom. Light into the cave comes from two sources: one is a small hole in the cave wall, precisely at the waterline, that is a meter and half in diameter, and used as the entranceway. The second source of light is a larger submerged hole lying directly below the entranceway, which is responsible for most of the lighting.

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