Piscine de la Butte-aux-Cailles

Paris has a natural hot spring, and you can swim in it year-round. The Piscine de la Butte-aux-Cailles is fed by a deep well that gushes water from 580 meters below the city at a constant 28°C. The well was discovered during drilling in the mid-19th century and finished in 1904 after years of geological and financial setbacks. Locals still arrive with empty bottles to fill from the spring- the water is drinkable straight from the ground.

Designed by Louis Bonnier and built between 1922 and 1924, this was the first pool in Paris to mandate that swimmers pass through showers (and a footbath!) before entering the basin. The separation of changing cabins from the pool itself was realized here for the first time in France. Inaugurated in 1924, the year of the Paris Olympics, it wasn’t the biggest or fanciest pool in the capital, but it was remarkable for its water supply and technical innovations. Today it remains one of the few swimming facilities to offer an outdoor Nordic pool heated to 28°C all year round. In a 21st-century twist, a data center now sits beneath the basins, providing additional heat to warm the water. Most cities bury their natural resources under concrete, but Paris turned its underground hot spring into a year-round swimming hole that’s still free to enter.