The Notre-Dame-Sous-Terre Chapel, accessible on rare guided tours, is one of the most mysterious sites on Mont-Saint-Michel. Everyone agrees that this is the oldest part of the monastery. Built at the beginning of the 10th century, the chapel was later used as the base for the western part of the nave of the Romanesque abbey church. Skeletons of pilgrims, prisoners, soldiers and clerics have been found there. Piously preserved but gradually forgotten over the centuries, Notre-Dame-Sous-Terre still holds many secrets.