Ryusendo Cave — Underground Crystal Blue Lake

Ryusendo Cave is one of Japan's three largest limestone caves, famous for its underground lakes of exceptional clarity and brilliant blue water. The main lake (第三地底湖, Third Underground Lake) is 98 meters deep with transparency exceeding 40 meters — the water appears crystal-clear blue due to its purity and depth. The cave system extends over 5,000 meters, though only 700 meters are open to tourists. The accessible route includes paved walkways, lighting, and viewing platforms overlooking three underground lakes. The cave maintains a constant 10°C temperature year-round. The lakes contain rare cave-dwelling fish and bats, and the cave walls feature spectacular limestone formations — stalactites, flowstone, and calcite crystals. The blue color of the water is entirely natural, caused by light filtering through ultra-pure water and reflecting off the limestone bottom. The cave is designated a National Natural Monument and is associated with Sanriku Fukko National Park.