Grotte du Chien, Natural cave and tourist attraction in Chamalières, France

Grotte du Chien is a cave in Chamalières whose entrance opens in basalt from lava flows of the petit puy de Dôme or the puy de Gravenoire. Carbon dioxide of deep origin, heavier than air, collects at the bottom of the cavity. A wooden walkway with a thin railing runs across the rock face above dark water, and a small rectangular sign stands on the rock above a simple bench.

The cave was first mentioned in the 18th century, when a miller stored wine there and called it "l'étouffis". In 1787 the writer Pierre Jean-Baptiste Legrand d'Aussy descended into it, fell ill and published an account that brought it national attention. Doctor Petit bought the cave in 1875 and ran it as a tourist site; it was exploited until 1986, then by Claude and Huguette Girard from 1994 to 2004, before the town of Chamalières acquired it.

The name refers to the Grotta del Cane at Lake Agnano near Naples, where dogs also suffocated in carbon dioxide. The cave was also called grotte Saint-Mart, after the hermit Martius of Royat, who lived around 450 and is said to have healed many people, including the father of Gregory of Tours.

The site is currently managed by the municipality of Chamalières and remains closed to visitors. This closure is due to safety concerns related to gas levels inside.

The bench inside the cave is called the "banc des belles-mères" (mothers-in-law' bench), supposedly installed so visitors could seat a mother-in-law they wished to be rid of. It sits where carbon dioxide, heavier than air, collects at the cave floor.