Much like the other similar establishments in the region, one is expected to wash thoroughly before entering the main baths. But this is especially important at the world's only #gravy onsen. Geothermal energy heats the warm, silky fluid to a relaxing 39 degrees, and the house chefs continually refresh the savory brew with locally grown vegetables and parts harvested from animals, kami, and yōkai attracted to the enticing odor.