One of the three largest limestone caverns in Japan, Ryūsendō Cave features expansive subterranean galleries arching over deep sunless lakes filled with breathtakingly clear blue water. The complex has been designated as a natural monument by the Japanese government since 1934, thanks not only to its remarkable size and enchanting aquatic features, but also because of the unique varieties of bats that call the cave home.

Surveys of Ryūsendō Cave began in 1920, and thus far, some 3600m (about 2 miles) have been explored and mapped, of which 700m (a half mile) can be visited by the public. Exploration is ongoing, however, and it is believed that the full extent of the cave is likely 5000m (3 miles) or more. Additionally, eight underground lakes have been found in Ryūsendō, fed by underground river of crystal-clear spring water constantly gushing forth from the bedrock. 

Three of these lakes are accessible to visitors and feature submerged lighting that illuminates that chambers with the brilliant blues and greens of the limestone-filtered waters—colors pleasingly saturated thanks to the clarity and depth of the waters. A fourth lake that is not included in the tour reaches an impressive depth of 120m (almost 400 feet), making it the deepest underground lake in Japan as well as one of the most transparent lakes in the world.

Elevated walkways, bridges, and stairs conduct visitors through a 30-minute tour that ranges from the water's surface to the cavern roof, where those intrepid enough can view the watery depths from amidst stalactites high above. While up there, one might even come face-to-face with some of the rare bats that populate the cave, including Brown long-eared bats, Eastern long-fingered bats, Hilgendorf's tube-nosed bats, and Greater horseshoe bats.