One of Tokyo's few true onsen, this chic spa complex draws natural hot-spring water from 1,700 metres below ground. You'll find indoor and outdoor baths, saunas, and a menu of add-on treatments including Korean-style whole-body exfoliation. It's a fascinating introduction to Japanese health and beauty rituals right in the middle of the city. After soaking, you emerge into the urban jungle feeling like a completely different person.