In the year 708 Mount Belenus, which until that period had formed a part of the mainland of Armorica, was suddenly detached from it by a terrible catastrophe which spread desolation over the country. The sea, flowing in with tempestuous fury, overpassed its limits, submerged the ancient forest, as well as the inhabited parts of the coast, and, except when the tide is out, made an island of the Mount.[15] It was in this same year of 708, in the reign of Childebert II., that St. Aubert, the first Bishop of Avranches, in obedience to a vision built there a church dedicated to the Archangel St. Michael, and at the same time founded a monastery of clerks regular, who replaced the two or three hermits who had formerly lived in seclusion on the Mount. This monastery acquired, later on, a fresh importance under the Dukes of Normandy. Duke Richard I. enlarged and made of it an abbey of the Order of St. Benedict. In 1002 or 1003, great part of the church and surrounding buildings being consumed by a fire which broke out, Duke Richard II. considerably enlarged as well as strengthened the foundation by the construction of the crypt, upon which the new edifice was raised. This crypt appears to be cut out of the solid rock, and is divided in two parts by a wall. Its low and vaulted roof is supported by massive pillars, round or square. A larger or grander subterranean vault does not perhaps exist, with its space of seventy metres in length by twelve in breadth, and its three aisles formed by about twenty pillars. The roof sustains the weight of two stories of building, the dormitory over the refectory, and the magnificent cloister over the Hall of the Knights.[16] The original church soon becoming too small to contain the numerous pilgrims who flocked thither, the construction of a new one was begun by the Abbot Raoul, who, in 1048, raised the four pillars and the arch of the great tower. The nave, and that part of the monastery called _La Merveille_, were built by his successor, Renaud. It was in 1091 that Henry, the youngest son of the Conqueror, was besieged in the fortress of Mont Saint-Michel by his brothers Robert and William. After the expulsion of the wretched John from Normandy, Abbot Jourdain wishing to preserve the Mount to the kings of England, Philip Augustus sent against him Guy de Thouars, who, after a lengthened siege, being unable to take it, set in on fire. It suffered severely from another conflagration in 1350, when struck by lightning during a terrible storm. The liberality of Philip de Valois restored the church and monastery to more than their former splendor.