Kameyama Onsen Hotel

Kameyama Onsen Hotel sits on the shore of Kameyama-ko (亀山湖), the largest lake in Chiba, and both of its indoor bathhouses look out over the water. The spring is the hotel's own, rising naturally rather than pumped, and it turns the water a deep chocolate brown from ancient plant matter dissolved into it over the centuries, leaving skin noticeably smooth. The men's bath keeps a second, unheated tub of raw spring water next to the main one, for switching between hot and cold. A private open-air bath with the same lake view, plus two smaller reservable semi-open-air baths (one built for small children, with a non-slip tatami floor, a raw cold-water tub, and a hundred rubber ducks), can all be booked by the hour. The hotel has been run by the same family since 1950 and is now on its third generation; the private open-air bath was the third-generation young master's long-held dream. The building is old-fashioned and a little worn in places, but regulars come back for the water and the staff rather than for polish, and the hotel is certified baby-and-toddler friendly, with kid-size tubs and bath chairs throughout.