The road through the Grotte de Mas d’Azil in France. Limestone karst is best known among engineering geologists for its distinctive range of geohazards, most frequent of which are new sinkholes developed within the soil cover, whereas collapses of rock into underlying caves are less common. Caves can rarely provide a benefit within karst, by providing ready-made tunnels. In the fold-range foothills along the northern side of the Pyrenees, in southern France, the Grotte de Mas d’Azil was cut by the River Arize through a ridge of Cretaceous limestone forming the core of a gentle syncline. The cave is 420 m long and up to 50 m wide, with a stable roof arched up to bedding planes in the strong limestone. Though known and occupied since Palaeolithic times, it now carries Route Nationale 119 along the bank of the underground river and safely above flood level. The Mas d’Azil is one of only a few cave tunnels around the world; these include the road through the Grotta di San Giovanni in Sardinia, Italy, the railway through Natural Tunnel in Virginia, USA, and a handful of roads through large caves in remote parts of China.

Photograph by: Tony Waltham, Geophotos.