When a wave hammers into a seashore wave, a weak part in a cliff is being shaved, and a cave is built. A cave in the sea where it's possible in this way is called sea cave. A copy of ceiling of the big sea cave is deformed, and Ryuguu Cave of a maggot is a skylight of about 50 meters of diameter. The one which opened. On the wall there is a skylight of Ryuguu Cave in all part in Izu, but even Naka is maximum, and which is a cave, submarine volcano A contrast with the blue seawater