Once upon a time, when bathtubs in private homes were a rarity, the great defining social institution of Japanese urban life was the relaxing sento: the local public bath. And if the sento was also an onsen—a thermal spring—with waters drawn from some mineral-rich underground supply, the delight was even greater. No more than a handful of such places survive in Tokyo, but the Oedo Onsen managed to tap a source some 4,600 feet below the bay. Visitors can choose from several indoor and outdoor pools, each with different temperatures and motifs—but remember that you must soap up and rinse off (including your hair) before you enter any of them. Follow your soak with a massage and a stroll through the food court—modeled after a street in Yoshiwara, the licensed red-light district of the Edo period—for sushi or noodles and beer. Charges include the rental of a yukata and a towel.