Aven d'Orgnac, Show cave in Ardèche, France

Aven d'Orgnac is a limestone cave system in the village of Orgnac-l'Aven in southern France. Three stacked chambers reach a combined depth of 121 meters (397 feet) and contain formations ranging from slender columns to broad curtains of flowstone.

On 19 August 1935, guided by villagers who knew the shaft as the aven du Bertras, Robert de Joly descended with four companions and explored as far as the Salle Rouge; the expedition lasted ten hours. The council voted funds for development in February 1936, work began in autumn 1938, and the cave opened on 11 July 1939. Explorers passed the chatière at the back of the Salle Nord in the 1960s, and Orgnac II, III and IV were found in 1965 and 1966.

The name comes from the Provençal word for sinkhole, because shepherds used to lose animals in these natural openings. The attached museum displays finds from the entire Ardèche river gorge, which people have lived in since the Stone Age.

Visitors walk down 700 steps during the hour-long tour and take an elevator back to the surface. Sturdy shoes and a light jacket make sense because the path can be slippery and it feels much cooler inside than outside.

The Salle Robert de Joly holds a sinter formation called the Buffet d'orgue, named for its resemblance to organ pipes; an urn with Robert de Joly's ashes sits at its centre. The cave's average temperature is 11 °C, and the deepest accessible point lies in the Salle rouge, where boreholes revealed a water-filled network 50 to 100 m below.