Salnitre Cave is in the town district of Collbató, 1 kilometre up from the town, at the foot of the Montserrat massif. It is a natural underground cavity, 530 metres long, making it the second most important in the Principality in terms of topography. The change of level inside is of some 20 metres, and the temperature, 14 ºC. Known since the 16th century, it was already explored at the end of the 18th century, but systematic exploration took place in the mid 19th century. One of its famous visitors was the architect Antoni Gaudí. It is said that he was so impressed by the wonders he saw in it, that it was from that time that stalactites and stalagmites began to inspire his marvellous work. In 1930, a lighting system was inaugurated, created by the engineer Carles Buïgas. After the Spanish Civil War the cave was abandoned for many years, but in 1985 it was opened again after two years of works. These improved its paths, lighting, accesses and parking, and restored areas sacked by collectors. Nearby there are other less important, but interesting, caves like the 150 metre long Freda (Cold) Cave.