Rivière souterraine du Heurt, Underground cave system in Senneville-sur-Fécamp, France.

The Rivière souterraine du Heurt is a karst spring and cave in the chalk of the Pays de Caux, at Senneville-sur-Fécamp. Its galleries carry a subterranean stream that resurges at the foot of the coastal slope, and the water has dissolved and enlarged the chalk fissures along its course.

The cave system took shape during the Quaternary period through erosion of limestone deposits. This geological process carved the underground passages that exist today.

The Heurt stream that resurges here is a tidal spring: its flow reverses with the rising tide of the English Channel, pushing seawater back into the passages. Villagers have long timed walks to the cliff foot by this rhythm, and the cave is known locally as a "fontaine" that breathes with the sea.

Access depends on water levels and seasonal conditions, so contact local authorities before visiting. Planning ahead ensures you can explore safely and appropriately.

The cave lies in the chalk of the Alabaster Coast, the same Cretaceous limestone that forms the cliffs at Étretat and Fécamp. The underground river emerges as a spring near Senneville-sur-Fécamp, and the water carries dissolved calcium carbonate picked up as it passes through the rock.