At the Grotta Giusti in the spa town of Montecatini Terme, Italy, an ancient thermal cave that acts as a natural steamroom has been attracting visitors for more than 150 years. The underground cave – thought to be 130 million years old – was first discovered in 1849, and was quickly expanded into a spa retreat once the water’s healing powers were realised. Composer Giuseppe Verdi was a regular visitor, and regarded it as “the eighth wonder of the world”.

Part of the Italian Hospitality Collection – which operates three other thermal hotels in the country – the 64-bedroom Grotta Giusti hotel is a 19th-century stone villa with delicately hand-painted ceiling frescoes and period decorations in the common areas.

The hotel’s outdoor thermal pool, set amidst slender cypress trees in the tranquil Tuscan countryside, is a pleasant 34 degrees Celsius. At one end of the pool, locals leisurely enjoy the hydrotherapy circuit, moving through a series of 40 hydro-massage stations. Everyone wears the required red-and-white swimming caps. From across the pool, their heads bob above the water like some strange insects, while the buzz from the low murmur of Italian reaches us in waves. The activity is relaxed and social in a way that I imagine has been passed on through generations. Every so often, everyone moves on to the next station.

These day spa guests are big part of Grotta Giusti’s business – 80,000 people each year pay the day rate to bathe in the pool’s mineral-rich waters – but the star here is the ancient underground cave, said to be the largest in Europe. Hotel guests pay an extra €20 for entrance to the cave, and day guests pay €40, but that doesn’t appear to be a deterrent.

“All guests in Grotta Giusti hotel pay for the grotto,” says Dr Nicola Angelo Fortunati, the hotel’s health expert. “The thermal cave is unique in Europe and people come to Grotta Giusti especially to experience it.”

Journey to the Inferno
This is precisely why I’ve come, stopping here between a day trip to Cinque Terre and our final destination in southern Tuscany. So, after basking in the outdoor thermal pools for a few hours, I head for the cave. At the entrance, I trade my cosy terrycloth spa robe for a thinner, hooded pullover version, designed to keep me more comfortable in the humid air. The hooded robe does the double-duty of making me feel like I’ve donned a monk’s habit or a Jedi robe and stepped into another time and place – a sense that is reinforced once I’m inside the otherworldly cave itself.

My pilgrimage to the cave’s inner belly begins along a well-lit path, which leads me underground down a gentle slope as I enter the first of three areas. Three regions referencing Dante’s Divine Comedy are each maintained at a different temperature: Paradiso (heaven) is the balmiest of the three; Purgatorio (purgatory) includes a Limbo pool at medium temperature, and Inferno (hell) reaches 98 per cent humidity.

Guests travel through the three realms in an underground wellness circuit that lasts around 50 minutes, slowly easing their way deeper into the heat and humidity. The cave also has a lake deep enough to swim and scuba dive. For an extra €100, guests can experience a floating watsu-style therapy that incorporates stretches and flexes in the thermal water, and the hotel’s spa also uses mud baths, inhalations, sprays and aerosol treatments, all of which incorporate thermal water and mud from the grotto.

The cave’s walls are rounded and dimpled like overgrown cauliflower and punctuated with stalactites and stalagmites – evidence of the mineral-rich waters within – but the path is even and well-lit and the rooms are high enough to stand in comfortably. The effect of the natural architecture, combined with the hazy steam and the shadowy light of the spotlights, is stunning. Along the way, loungers are placed to encourage you to sit and relax, which I do, adjusting slowly to the heat and the quiet, dim space before descending to the ultimate destination: the Inferno.

A truly mindful state
The Inferno is actually much more pleasant than the name implies – though I’m a Miami girl at heart and love a dose of heat and humidity. Signs at the entrance read ‘Il silenzio aiuta il relax – Silence helps relaxation’ and the cave draws into itself here, with a lower ceiling and smaller nooks, creating a sense of intimacy and privacy.

My fellow Jedi remain shrouded beneath their robe hoods to stay protected from the dripping condensation, and seem to take their instructions seriously: only the occasional whisper hangs in the soupy air, creating a church-like quiet. The silence, the monk robes and muted lighting combine to create a feeling of hushed reverence. Reclining chairs are situated throughout the room, and I settle into one, relaxing as the vapours enter my lungs. The density of the air means it’s virtually impossible to do anything but slow down my breathing, which instantly calms my mind, and the only sound is the rhythmic, hypnotic dripping of the moisture pooling off the cave’s walls.

While I don’t spend the full 50 minutes on the circuit, I spend more time in my meditative state underground than I would in a traditional steamroom, simply relaxing in a quiet, serene space. The dimly lit Inferno and the cavernous silence have taken me to a truly mindful state.

I’ve been told the cave has a high concentration of negative ions, and the aerosolized water running through the cave is rich in calcium, bicarbonate, sulphur and magnesium, said to ease inflammatory respiratory diseases as well as osteoarthritis and skin disorders, such as eczema. I suffer from neither, but my husband’s mild eczema was miraculously milder – in fact, almost non-existent – after our visit.

When I emerge from the depths of the cave, I feel I’ve been transported – my skin is dewy with thermal moisture and my eyes take a few seconds to adjust to the real world. An achy shoulder that normally nags me has gone silent, and I’m quite sure the magnesium has seeped through my pores and given me an energy and mood boost as well. Either that – or maybe I’m just feeling slightly euphoric at having made the trip through heaven and purgatory, and to the gates of hell and back.