Don't miss the Puente del Inca, a natural arch that forms a bridge over the Río de Las Cuevas. It has developed over millennia through the slow depositing of water that is rich in limestone and minerals, in former times renowned for its therapeutic properties (34-38°C at the source). Its colour comes from the water's high iron (rust) and sulphur (yellow) content. The building gradually being buried beneath bright yellow deposits is a former spa hotel, which closed in 1965.