Ten places where the hot water is the reason to go: Ottoman bathhouses, Japanese onsen towns, Roman pools and geothermal lagoons, ranked, with the bath to book first.
We ranked these ten by the water and the bathhouse: the source, the setting and the bathing culture built around it, and then by how easily a first-time visitor can walk in and use it.
No city bathes like Budapest. Széchenyi, open since 1913, keeps its outdoor thermal pool at 36 to 38°C year round, and across the river Rudas still runs its 16th-century Turkish bath: an octagonal pool under a dome on eight pillars.
The onsen town Japanese travellers rate first, built around the Yubatake, a steaming field of wooden channels that cools the water in the town centre. Kusatsu has Japan's largest free-flowing volume, and the water is strongly acidic at pH 2.1.
Japan's largest output of hot spring water, and the widest range of ways to use it: sand baths, mud baths, steam rooms and the coloured Jigoku you look at but never enter. Takegawara, founded in 1879, buries you in warm sand in its 1938 building.
The easiest serious onsen trip in Japan: the Romancecar reaches Hakone-Yumoto from Shinjuku in about 80 minutes, and the day baths take walk-ins. Stay one night in a ryokan with its own bath, and do the loop to Lake Ashi the next morning.
White travertine terraces laid down by the springs, with the Roman spa city of Hierapolis on top. You walk the terraces barefoot, but the swim is the Antique Pool, warm water over toppled marble columns, charged on its own ticket.
Milky-blue geothermal seawater from the Svartsengi field, set in black lava, at 37 to 39°C. It sits near Keflavík airport, so most visitors bathe on arrival or departure day; check the operator's notices first, as nearby eruptions have closed it at short notice.
One of the Great Spas of Europe, and Friedrichsbad, opened in 1877, is still the bath to do. Its Roman-Irish ritual runs through 17 stations of warm air, steam and thermal pools, textile-free except on Wednesdays and Saturdays.
A town on a live geothermal field, where the steam rises from the parks and the Te Arawa hosts explain what you are seeing. Soak at the Polynesian Spa on the lake edge at dusk, in two springs: acidic Priest (pH 3.1) and alkaline Rachel (pH 8.9).
The city is named for its warm water, from tbili, Georgian for warm, and the brick-domed sulphur baths of Abanotubani still sit under the old town. Book a private room and the scrub; Chreli Abano, behind the blue mosaic front, is the one to know.
The great drinking-cure town: you sip the springs from a spouted cup under five colonnades rather than bathe in them. The Vřídlo shoots up to 12 metres at 73°C in its own hall, and the Hotel Thermal's outdoor pool on its hillside terrace is the swim.
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