Bagni San Filippo: the White Canyon and Wild Thermal Springs of Tuscany

A limestone canyon in the heart of Tuscany

Along the road that descends from Siena towards Monte Amiata, hidden among the chestnuts and oaks of a dense wood, a spectacle opens up that seems to belong to another planet. The Fosso Bianco at Bagni San Filippo is a cascade of pure white limestone formations, shaped over centuries by thermal water that emerges at nearly 48 degrees. Steam rises through the trees, the scent of sulphur mingles with the smell of resin, and the silence is broken only by the gurgling of water sliding over the travertine formations.

We are in the municipality of Castiglione d'Orcia, in the province of Siena, at an altitude of about 524 metres. A place the Romans knew well — the thermal springs were already frequented in the imperial era — and one that in the Middle Ages became a stop for pilgrims heading to Rome along the Via Francigena. San Filippo Benizi, the saint who gives the borgo its name, is said to have taken refuge here in 1269 to escape election to the papacy.

The formations of the Fosso Bianco

The main formation is known as the Balena Bianca — the White Whale — a calcium carbonate deposit more than ten metres long that recalls the back of a beached cetacean. Water gushes from the sulphurous spring at high temperature and, as it cools, deposits successive layers of travertine, creating natural pools where you can immerse yourself. The pools vary in temperature: those closest to the source reach 48°C, while those downstream settle at 36–38°C, ideal for a long soak.

The water is rich in sulphates, calcium and magnesium bicarbonates, and a significant component of hydrogen sulphide — the characteristic egg smell you notice even from the path. These properties make it particularly beneficial for skin conditions, rheumatism, and respiratory ailments.

The path to get there

From the small car park along the SP61 (coordinates 42.9292°N, 11.6975°E), a dirt path descends into the wood for about 500 metres. The route goes downhill, not demanding but slippery in wet weather. After five minutes of walking you reach the first stretch of the fosso, where the white formations begin to appear among the vegetation. Continuing for another two hundred metres brings you to the Balena Bianca and the main pools.

Practical information

Access and costs

The Fosso Bianco thermal springs are completely free and open access, twenty-four hours a day. There are no facilities, changing rooms, or toilets. The borgo of Bagni San Filippo, a few minutes' walk away, offers a couple of bars and restaurants where you can change and eat. There is also a private thermal establishment in the centre of the village, the Terme San Filippo Hotel, with pools and treatments at a charge.

When to go

The best period runs from September to May, avoiding the summer months when visitor numbers increase significantly, especially at weekends. The autumn and spring months offer the most evocative contrast: steam rising from the hot water in cool morning air creates an almost otherworldly atmosphere. In winter, with snow on the surrounding trees, bathing in the steaming waters becomes an unforgettable experience.

What to bring

• A dark-coloured towel (limestone and sulphur stain light fabrics)
• Water shoes or sandals with non-slip soles for walking on the formations
• A swimming costume you don't mind ruining (sulphur can discolour it)
• Drinking water and a snack
• A bag for rubbish — there are no bins
• In winter, a warm bathrobe for the walk back to the car park

Precautions

The pools closest to the spring can be very hot: always test the temperature before immersing yourself. The bottom is uneven and slippery. Avoid entering with open wounds. Hydrogen sulphide, at high concentrations near the spring, can cause discomfort to those with respiratory problems: if nausea or headache occurs, move away from the source.

Nearby

Bagni San Filippo occupies a strategic position for exploring the Val d'Orcia and Monte Amiata. Thirty minutes by car brings you to Pienza, the Renaissance borgo of Pope Pius II, and to Castiglione d'Orcia itself with its medieval fortress. Monte Amiata, with its trails through ancient beech forests, offers trekking in every season. Heading south, the Abbey of San Salvatore, founded in 743 and one of Tuscany's oldest monasteries, is worth a stop.

For lunch, the borgo has the Trattoria La Fonte, where you can enjoy pici all'aglione and stewed wild boar — dishes from the Amiata tradition at modest prices. Those seeking a more complete experience can stay at the village's thermal hotel, which also offers access to the internal hot springs.

A place that is always changing

The Fosso Bianco is never quite the same. The formations grow and change shape with the passing months, new pools form where the water finds an opening, and the landscape shifts with a geological slowness that is, on a human scale, surprisingly rapid. To return to Bagni San Filippo after a few years is to find a familiar yet different place, where nature continues its silent work as sculptor, indifferent to the visitors who pass through.