Hoshi Onsen Chojukan

A lone Meiji-era inn where the water bubbles up through the stones beneath you — one of Japan's great baths, if you can handle mixed bathing, no air conditioning and autumn stink bugs

Chojukan is the only inn at Hoshi Onsen, at the end of a mountain road in northern Gunma, and its reputation rests on one room: Hoshi-no-yu, an 1895 wooden bathhall where the hot spring rises straight up through the pebbles under you, never touching the air. Japanese guests score the baths higher than anything else — 4.75 on one major booking site and 4.71 across 518 reviews on another — and the 1875 main building, the bathhall and the 1940 annex are all registered cultural properties. Guests love the hearth kept burning all year, the sound of the river and being able to use the bath all night: as one put it, that is 'the privilege of staying'. Two things put people off. Hoshi-no-yu is mixed-gender, with women-only time from 20:00 to 22:00 and no swimwear or towels allowed; several women found it daunting, and recent reviews mention men staring and even walking in during women's hours. And these are old buildings: many rooms have only a fan, walls are thin, there are stairs and no lift, and autumn reviews count stink bugs by the dozen. Food is hearty mountain cooking — carp, ayu, deer, Joshu beef sukiyaki — and scores lowest, with complaints about dishes arriving all at once.