Capri's Blue Grotto is known throughout the world for its size, the intense blue tones of its interior and the magical silvery light which emanates from the objects immersed in its waters. Visitors can reach the Grotta Azzurra either by bus or taxi from Anacapri; or by boat from Marina Grande in one of the tour boats which stop at the entrance of the cave during the trip around the island. The interior is suffused with magnificent blue tones. The vault, known as the Duomo Azzurro, has an average height of 7 meters rising to 14 towards the back of cave.
The blue coloring of the Grotta Azzurra is created by the daylight which enters via an underwater opening located immediately below the entrance to the cave. The light is filtered by the water which absorbs the red tones, leaving only the blue ones to pass into the cave. A second phenomenon creates the silver appearance of the objects immersed in the water: given that the index of refraction of the bubbles of air which adhere to the surface of the objects is different to that of the water, the light is allowed to egress. It is believed that, in the Roman period, under the rule of Tiberius, the interior of the Grotta Azzurra was used as a marine nymphaeum. There have been those who imagined the cave as the habitat of Nereidi or of Sirens or believed it to be the realm of devils who bewitched all who dared to enter.
