Réserve naturelle régionale du cratère du Mont-Bar, Regional nature reserve in Allègre, France.

The réserve naturelle régionale du cratère du Mont-Bar occupies a 500-metre-wide crater, 40 metres deep, on a strombolian cone rising to 1,172 metres in Allègre, Haute-Loire. The crater floor holds a peat bog of about 4 hectares, ringed by dense trees and shrubs along the rim. Clay-altered projections sealed the crater bottom, letting a lake form that vegetation gradually turned into bog.

The volcanic crater formed more than 790,000 years ago through strombolian eruptions that shaped the landscape. Archaeological excavations in the 1800s uncovered ancient artifacts, revealing that people have known and valued this location for thousands of years.

Local legends link the bog to Roman-era mock naval battles, a story retold by George Sand in her novel Jean de la Roche; the writer came here to collect plants for her herbarium. A second tradition holds that Gallic druids made offerings at the site, and excavations in 1821 brought up gold bracelets and medals now kept at the Crozatier museum in Le Puy-en-Velay.

The crater floor is reached on foot; the bog itself is not crossed, and the crater rim is followed on paths. The ditch dug by local farmers in 1940 to drain the former crater lake is no longer maintained and is slowly silting up, so the wettest ground stays wet. Waterproof footwear helps after rain.

The crater's volcanic projections weathered into clay that sealed the floor, letting a lake form and then turn into peat bog. A drainage ditch dug by local farmers in 1940 is no longer maintained; it is silting up, water is rising slightly, and the bog may slowly return to a crater lake.