It was one of those intensely cold March days in Tuscany, where the dying winter puts on its best effort to freeze bones for the last time, and my new boyfriend and I were winding along roads off the motorway south of Siena. It was the beginning of our story and I wondered where he was taking me. We meandered through the perfectly ordered Chianti countryside studded with elegant cypress trees and into a wilder landscape of thickly wooded hills rucking up dramatically into the sky – big country superimposed on a small, groomed land.

The day was drawing to a close. Below us a river wound its way through a verdant copse. We scrambled down the sloping dirt track to it, the remains of a Roman wall on our left, a high bridge arcing above us to the right. Below, forming an opaque pool by the river, was a natural thermal bath fed by hot springs trickling down a formation of rocks, steam rising into the darkening sky. We stripped off – to our knickers as we hadn't brought swimwear, having gone out just for lunch – and ran through the icy air into the water, relieved to lie down and have it lap our chilled shoulders. It was as warm as a bath and smelt of rotten eggs. My Italian man explained that this was the result of the main mineral present in the water – sulphur – and that, hard as it was on the nose, it was the surest way to baby-soft skin. And we lay in the water as the sun went down in a blaze of colours, the vapour snaking into the frigid air as the moon rose and the expansive Tuscan sky was stitched with layers of stars.

This was my introduction to Petriolo, one of Tuscany's natural hot springs, and although the boyfriend didn't last, my love for the wild thermal baths that lurk in the countryside remains to this day.

Tuscany is rich in hot natural springs that bubble out of the ground filled with minerals and the heat of the earth. The springs have been used for their curative properties since the time of the Romans, who believed that the sulphurous water came from Hades, the underworld. Now the major hot springs are used by spa hotels and medical centres. But best of all are the wild hot springs which still pepper the countryside, free and untamed – if you know where to look.

Petriolo

Twenty minutes south of Siena are the Bagni di Petriolo, hot springs that were popular with the Romans, and later with the Medici family. The pools are located below ruined walls dating from 404AD, the only remnants of fortified Roman thermal baths. Petriolo is signposted from the main Siena to Grosseto road – then follow your nose down to the picturesque river Farma to find it.

The springs gush down into a series of small baths, climaxing into a large pool where people ease aching shoulders by sitting under the torrents of water. At 43C the temperature is perfect for cold days, and the pools collect water at different temperatures – there is the river alongside to dunk into for a blast of cold water. The locals gather clay from the riverbanks which they dry and pulverise to a fine powder. When mixed with the thermal waters, it turns into the sort of mineral mud sold at spas for a fortune, but at Petriolo it's free. The water is said to cure everything from colds to arthritis and to soften skin.

Saturnia

Italy's best-known hot spring is at Saturnia in the wild Maremma area of southern Tuscany, about an hour south of Siena. It is a dramatic landscape of thickly wooded hills and valleys, olive groves, rows of vines and acres of wheat fields, the horizon riveted by cypresses.

Legend has it that the Roman god of the harvest, Saturn, lost his temper with war-hungry men and sent a lightning bolt that split the earth. Hot sulphuric water flowed from the rift over the people, calming them down. The main falls are easy to find: not far from the town's main spa, a dirt track brings you to a field where you can park and change before heading down to the springs – a series of rock pools that have been turned white by the minerals in the water, which falls in a succession of cascades into the pools.

The water is about 37C and instantly relaxes the body, releasing aches and pains. The minerals here are sulphur, calcium and carbon, good for skin conditions and aching joints. When it gets too hot, there is the river Albegna alongside to dip into to cool down.

San Filippo

Located 40 minutes south-east of Siena, between the volcanic peaks of Mount Amiata, the highest mountain in Tuscany, and the deeply wooded Orcia valley, San Filippo takes its name from the Florentine hermit who retreated here in the 13th century. It is also known for its five hot springs, which gurgle out of the ground at 52C and are rich in sulphur, calcium, magnesium and sulphur bicarbonate, believed to be good for skin conditions, respiratory disorders and joint and bone problems. On the approach to the town a small wooden bridge on the right leads into the woods to a bucolic scene of fluttering butterflies, buzzing dragonflies and frogs hopping into the green water filling the pools that the river flows into. Most stunning of all is the Fosso Bianco – a series of calcareous rock formations that hang over the water in a series of stalactites. The water used to pour down here – though it has now been diverted to the spa in the town. But follow the path through dappled daylight and you come to pool after pool under waterfalls of opaquely white water, set below the spa above, from where the water gushes down. San Filippo's milky mineral water flows through the pools and along the river, banked by bamboo, for miles.

Where to stay

The nicest place to stay within easy striking distance of all three wild hot springs, is Castello di Vicarello (+39 0564 990718), a renovated 12th-century castle set in the mountains of La Maremma, with 360 degree views of the hills and valleys spread out below. There are two swimming pools, a spa with a hot tub overlooking the wooded valleys and delicious organic food and wine from its own estate. But beauty costs, and doubles start at €370 per night including breakfast.

Try La Locanda del Loggiato (Piazza del Moretto, Bagno Vignoni, +39 0577 888925), a charming B&B set in a 13th-century building in a spa village within striking distance of all three wild hot springs. Doubles from €130.