You'll change into a yukata, wander down to the beach, and lie down while women with shovels bury you in naturally heated volcanic sand under a canopy of bamboo slat blinds. Reactions range from panic to euphoria — the warm, heavy weight pressing against your body is unlike anything else. Ten minutes is said to purge impurities, but many people stay longer, letting the heat sink into their bones. When you emerge, steaming and tingling, you'll head upstairs to soak in the onsen bath. It's one of Japan's most singular wellness experiences, and it's been happening here for centuries.