Gyokusendo Cave stretches for five kilometres beneath southern Okinawa, making it the second-longest cave in Japan. About 850 metres are open to the public via a lit walkway, threading through chambers where stalactites and stalagmites grow in forms that range from needle-thin to column-like, some meeting in the middle to form pillars. The cave maintains a constant temperature and is noticeably cool compared to the subtropical warmth outside — a genuine relief in summer.
The cave is the centrepiece of the Okinawa World complex in Nanjo, which also includes a reconstructed Ryukyuan village with traditional craft demonstrations, a habu snake museum (the habu is Okinawa's notorious pit viper, large and venomous enough to be genuinely feared), and a Ryukyu glass workshop. The complex can feel theme-park-ish, but the cave itself is the real thing: one of Okinawa's most impressive natural sites, and worth the entrance fee on its own.