Seven naturally occurring bubble baths to warm your bones .

Every continent on Earth is home to rumbling geothermal activity. Along the fault lines between tectonic plates, the water heated by the planet’s mantle can seep to the surface in no more than a trickle, but sometimes it thrusts its way up into a vertical water cannon. Most of these sites are too dangerous for a dip: in Yellowstone National Park in Wyoming, US, the springs are so hot you could boil an egg in seconds. The sweet spot is where cold springs mix with hot, creating the planet’s naturally occurring, mineral-rich Jacuzzis. And is there anything finer than lying back and luxuriating in a hot outdoor spring? MR PORTER bathes its way around the globe to bring you the very best.

Built in 1923, this picturesque pool is more popular with locals than visitors, and dates back to when swimming practice became compulsory for schoolchildren. A simple hot-springs pool, lodged on the edge of dark, snow-capped hills, it is fringed with grey-green vegetation. Solely staffed by volunteers, who did a heroic job of cleaning it up after the Eyjafjallajökull eruption in 2010, the pool’s lifeguards are in short supply, and you won’t find any bar or cafe. But what you will find is one of the most beautiful natural backdrops in southern Iceland to take in.

**_Where to stay: _**for an upscale Icelandic getaway, Hotel Rangá offers an intimate, cosy space.

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The Unesco World Heritage site of Pamukkale, in Turkey’s southwestern Denizli Province, translates literally as “cotton castle”. The moniker comes from the terraces of dazzling white travertine, which spills over a sheer cliff like a frozen river. Amid the dazzling white rock, hot pools have formed. The finest is the Sacred Pool, whose water reaches a cockle-warming 35ºC. After a visit to the adjacent Hierapolis, the ancient Byzantine spa city, there’s no better place to soak in all of its history.

_Where to stay: _Richmond Pamukkale Thermal Hotel, a 10-minute drive away, this sleek resort sits among lush gardens on sun-scorched hills.

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Unlike Seljavallalaug, the Blue Lagoon is a major tourist magnet. Just under an hours’ drive from Reykjavik, this 800-year-old lava field has been a draw since the 1980s. Its towers of steam clouds and burnt-umber mountain backdrop are peerlessly beautiful. The lagoon is completely man-made and its mineral-rich, milky blue waters are said to sooth psoriasis. In 2015 alone, more than 600,000 visitors plunged into the near 40ºC water, making it one of the country’s most popular attractions. Lines often curl out of the entrance, so pre-booking is a necessity, but despite the crowds it is possible to find a secluded cove in its nine million litres of geothermal seawater.

_Where to stay: _billed as Iceland's first five-star hotel, Diamond Suites is the best boutique option.

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If there’s any truth in the Etruscan legend of how Terme di Saturnia came to be, it’s little wonder the pools keep so balmy. Legend has it that it was hit by an errant lightning bolt during a fraternal tiff between Jupiter and Saturn. But however it came to be, though, these densely wooded springs, situated two hours south of Siena, are well worth a visit. The gushing water (at a rate of 500 litres per second) has a temperature of 37ºC and carries a wealth of health-giving minerals such as carbon, sulphur and calcium, to name but three. The waterfall-fed pools have built up a striking white sediment over the centuries. Beautiful, bucolic and begging to be bathed in.

Where to stay: Terme di Saturnia spa hotel has its own private hot pools on site.

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Icy Alaska is unfolding as a hot destination for the bold and intrepid. The Last Frontier is still a vast swathe of glacial unknown, but is getting more accessible with new flight routes. The springs here became popular 100 years ago when miners came to take the waters for their aches and pains. As more people made the trek – it’s 55 miles from the Interior’s largest city, Fairbanks – a resort and ice museum were built, which you can still visit today. The journey to the springs is still very much part of the experience. As you drive parallel to the Chena River, with its numerous fishing and picnic spots, you can gaze at the electric watercolour of the aurora borealis and take a seat in one of the outdoor tubs to get some respite from the below-zero air temperatures, which, in January, can hit -25ºC.

Where to stay: Tonglen Lake. Right on the edge of Denali National Park in the main lodge or one of the cosy cabins.

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Beppu is one of the centres of onsen (hot springs) culture in Japan and its Jigoku Meguri pools, meaning depths of hell, are its most famous draw. This circuit of eight different hot springs are spread over two locations northwest of the city centre. The smaller selection, located four miles from the town, is the most interesting. Tatsumaki Jigoku, a tiny pool bordered by moss-strewn stone, is heavenly with a geyser erupting every 30-40 minutes. Nearby is the scarlet-coloured Chi-no-ike Jigoku pool – hell on earth.

Where to stay: Terrace Midoubaru is a serene hideaway boasting floor-to-ceiling windows with views of the sea or the mountains.

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The mineral springs in Banff, Alberta, kicked off Canada’s National Park programme. In 1880, two Canadian railway men attempted to commercialise the springs before the government stepped in to protect the site. There is now 188,000 square miles of land, across 13 provinces and territories, set aside for conservation. On the northeast flank is the aptly named Sulphur Mountain, where the thrust fault is full of calm Tiffany-blue water. While you can no longer swim here, there is still a museum with the old bathhouse to see. For a dip, you need to head three miles away to the Banff Upper Hot Springs.

Where to stay: The Premier Lodge at Cathedral Mountain Lodge has pebble-stone fireplaces and painted skies on the walls.

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