Blue Cave is the most celebrated attraction of the Greek island of Kastellorizo. The cave lies on the SE coast of the island, and is named by the inhabitants phokialí (Greek for seal's refuge). Being 40–50 m long, 25–30 m wide, and 20–25 m high, it is much larger than the world famous Grotta Azzurra on Capri, Italy. The light of the sun, refracted through the sea, is reflected in the interior of the Grotto, thereby producing a dazzling blue colour. The Cave can be visited only by boat and, since the entrance is barely one metre high above sea level, only under calm sea conditions: the best time of the day to visit is early in the morning, when the sun is still low.
Blue Cave (Kastellorizo) Wikipedia