La Gruta Spa

Along the highway between San Miguel de Allende and Dolores Hidalgo, natural thermal water rising from the region's ancient volcanic activity feeds a string of balnearios, of which La Gruta is among the most distinctive. Its signature feature is a short tunnel that opens into a partly enclosed cave pool, where the confined space concentrates the mineral-rich steam for what regulars describe as a natural sauna effect. Outside the cave, several open-air thermal pools set among gardens round out the visit -- OpenStreetMap records the site simply as a natural hot spring, and it has operated for decades as one of the region's most distinctive low-key thermal escapes, open Wednesday through Sunday.