The chief field, known as the Great Pyramid Field, begins almost opposite Cairo, at Abu Roash, where is the pyramid of Dadefra of the IVth Dynasty. The Pyramid is a tomb and the sole surviving part of the elaborate structure which the Egyptians of the Pyramid period devised for the accomplishment of this end of securing the duration of the personality of its owner. Through the Classical period, the entrance into the subterranean passages of the Great Pyramid was well known; but the knowledge had been lost by the time of the Arab Conquest, and the Khalif Mamun had laboriously to quarry his way through the masonry into the actual passages, leaving behind him the great hole, which is called Mamuns Hole. Yet the cunning and the skill of the XIIth Dynasty architects and masons proved as helpless as the massive power of the IVth Dynasty to protect the dead monarchs from the ravages.