The single most photographed thing in southern Tuscany is this string of milky-white limestone pools, where 37°C sulphur-rich water tumbles down a terraced rock face. The water has been bubbling out of the ground at the same temperature and flow rate for at least 2,000 years, and the calcium carbonate it carries has built up the white travertine basins you sit in.
Access is free and the site is open 24 hours a day; the most magical times are sunrise and after dark, when the warmth of the water meets the chill of the air and the whole valley fills with steam. There are basic changing huts and free parking, but no lockers, food, or coffee on site — bring sandals, a towel, and a torch if you come at night. The smell of sulphur takes some getting used to; jewellery will tarnish.