Saturnia's hot springs in Tuscany's Maremma region are a free, open-air spa formed by naturally occurring travertine cascades. The sulfurous water holds a constant 37.5°C year-round, making it swimmable in any season. The Cascate del Mulino (Mill Falls) section is the most famous, with a series of stepped travertine pools. It's about an hour's drive from Grosseto, reached on foot from a roadside parking area.
