Gyokusendo Cave in Okinawa is a hidden gem beneath the sunny paradise of Japan. Discovered in 1967, this five-kilometer-long cave boasts stunning rock formations, crystal-clear pools of water, and an underground river. Visitors can explore the cave via an 890-meter steel walkway, marveling at the stalactites and stalagmites that have been forming for thousands of years. With psychedelic lighting and the option to go off the beaten path on special tours, Gyokusendo Cave offers a cool and mysterious escape from the heat above ground.