Ryuodo Cave is the largest lava rock cave on Sado Island of Niigata Prefecture. It is one of the Kotoura caves on the island, it is also sometimes called “Sado’s Blue Grotto.” This nickname comes from the color of the water: on clear days with mild weather, the reflection of the sunlight hitting the water creates a stunning shade of blue. There are kayak tours to the cave on which visitors can actually go inside and enjoy the views, and there are scuba diving spots nearby as well.