One of the world’s most picturesque places to soak, Pamukkale (“cotton castle” in Turkish) is a UNESCO World Heritage site featuring terraces of stunning white travertine pools shaped over eons by cascading calcium-rich water. Though many of the pools are now dry, bathers can still access the 96-degree, softly effervescent water, which was believed in antiquity to cure digestive and circulatory issues, as well as diseases of the eyes and skin.

Though the mineralized water here has an ancient source — the nearby St. Peter spring has been visited since the Bronze Age — the remote Alpine village of Vals’ distinctive spa complex dates only to 1996, when renowned architect Peter Zumthor completed a minimalist masterpiece constructed from 60,000 slabs of local quartzite. Stay at one of the two onsite hotels and spend your days marinating in the meditative austerity of Vals’ steaming thermal pools.

Detractors say that the Blue Lagoon is overcrowded and over-hyped. But it is still unique: High levels of silica and algae give the Blue Lagoon’s water an otherworldly milky-blue hue and render it supremely healing for the skin. The Retreat, a boutique hotel that opened in 2019, offers an ultra-private vantage point, with its own secluded stretch of lagoon, a luxurious spa, and glass-walled guest rooms that look out onto a lunar landscape of of volcanic rock and green moss.

The three natural pools at Castle Hot Springs, each a different temperature and accessed at the apex of a short hike, remain the resort's heart and soul. Immersion in the magnesium- and lithium-dense water delivers almost instant Zen, and best of all, the springs are accessible to guests 24 hours a day, so one can soak in solitude while gazing up at the stars. The property is as beautiful as it is historic— JFK even checked in for 3 months to rehabilitate after WWII.

Entrance to this museum lets you get up close to the original Roman soaking pool, encircled by ruins of a temple to Minerva, goddess of wisdom. Though brave souls can taste the eggy-smelling Bath water at the nearby Pump Room, there are only two ways for a modern bod to fully submerge: at the Thermae Bath Spa, which can get crowded, or via a stay at the Gainsborough Bath Spa, a 5-star hotel providing private access to the same water that eased the aches of centurions 2,000 years ago.

Volcanic activity has produced some 27,000 onsen across Japan, and there are nearly 3,000 towns dedicated to their enjoyment. Kurokawa, on Kyushu, Japan’s southernmost island, is the most beautiful, a fairytale-like town surrounded by lush forests. Visitors stay in traditional wooden ryokan inns and shuffle between the town’s 30 steaming onsens in yukata robes. Purchase a day pass for onsen-hopping, and dip in and out of the various pools, some in caves, others tucked away in bamboo groves.

Budapest has numerous medicinal mineral springs scattered throughout the city. Most famous is Szechenyi, but even more beautiful and less crowded is Gellert, which is attached to an Art-Nouveau hotel built in 1918. Encompassing 10 geothermally heated pools of various temperatures, Gellert is flush with the sort of exquisite details—turquoise ceramic tiles, frescoed dome ceilings, intricately carved stone columns and statuary—that invite you to absorb as much history as mineral goodness.

Esalen, a non-profit founded in 1962 according to the principles of Aldous Huxley’s Human Potential Movement, has drawn the likes of Ray Bradbury, Buckminster Fuller, Ansel Adams, and Bob Dylan—and you can bet that they all dropped their drawers for a dip in the property’s hot springs. The four pools, fed with mineral water from underground springs long ago held sacred by the indigenous Esselen tribe, are situated on plunging bluffs dramatically overlooking the Pacific Ocean.