Notre-Dame-Sous-Terre is a Carolingian church with two naves, each ending in one small sanctuary. It was only rediscovered at the end of the 19th century because it was buried under later buildings. The architect Yves-Marie Froidevaux (chief curator of the ‘Monuments Historiques’ in charge of the Mont from 1957 to 1983) is credited with having uncovered it by destroying a retaining wall. It stands on the site of the primitive oratory built by Aubert, the bishop of Avranches, in 708.