The natural geothermal forces on the island of Kyushu heat not only the water, but also the sand.
Beppu is home to several famous sand bathing spots known as sunamushi in Japanese. On the beach or in a bathhouse, visitors don a cotton bathrobe and lie on the sand while attendees pour in more hot sand to bury it up to their neck. After a 10-15 minute rep, you can clean up and head to the more traditional bathrooms. Sand apparently has great benefits for circulation.