The Grotta Giusti is the reason most visitors come to Monsummano. A natural limestone cave was discovered in the nineteenth century with mineral-rich steam at 34–36°C rising through its three connected chambers — named Paradise, Purgatory and Hell, in increasing order of warmth. You wear a robe, drink water, and walk slowly through the chambers for forty-five minutes to an hour before stepping out into the outdoor thermal pool.
The cave is now part of an Italian Hospitality Collection hotel, but day passes are easy to book. Weekdays are quieter and noticeably cheaper than weekends; book ahead in autumn and winter, when the contrast between the warm caves and cool outside air is at its best. The treatment is genuinely unusual — closer to a Roman caldarium experience than a modern spa — and the gardens around the cave entrance are pleasant to wander afterwards.