it’s located in a rocky cleft in the seashore of lovely little Shikine-jima, an island only a few hours’ ferry ride from downtown Tokyo. The pools are formed by the seaside rocks and it’s one of those onsen that only works when the tide is right. You can spend a few lovely hours here watching the Pacific rollers crashing on the rocks. And, there are two other excellent onsen on the island when you get tired of this one.