A typical characteristic of limestone is that is easily dissolves in water. Therefore, underground rivers can create huge caves in limestone. When the roof of such a cave has collapsed, a "sinkhole" or "doline" is visible at the surface. "Dolines" may also be created at the surface when water is collected in depressions.

The picture shows a doline near the village of "Drigas", at the "Causse Méjean".