Ibusuki Sunamushi Kaikan Saraku
Pay at the entrance, change into the provided yukata and wander down to the beach where, under a canopy of bamboo slat blinds, women with shovels bury you in hot volcanic sand. Reactions range from panic to euphoria. It's said that 10 minutes will get rid of impurities, but many stay longer. When you're through, head back up to soak in the onsen.