On the train ride from Narita airport to Tokyo Station, at the end of an 11-hour flight, the thought that I would soon be soaking in a hot spring buoyed my tired spirits. There are more than 2,000 natural hot-spring baths, known as onsen, and my partner and I planned to visit one at each stop along our journey. Our first hotel, Hoshinoya Tokyo, is billed as the only true onsen ryokan (hot-spring inn) in central Tokyo. This is somewhat surprising considering the country’s extensive bathing culture, but the geothermal waters here lie far below, and Hoshinoya had to drill a kilometer-deep well to tap into them.