Immerse yourself in another world, that of "one of the longest subterranean river networks" in the world, after Yucatán (40km/25mi!) in Mexico. A tour of this vertiginous chasm starts with a 500m/547yd boat ride down its mysterious river and continues on foot through vast caverns decorated with gigantic limestone concretions. The Padirac chasm remains a must for those keen to explore the Causse de Gramat.