Mr. Dupré bases his conclusion upon the fact that the well is merely a rough hole and the grotto an irregular chamber, without any evidence of the architectural precision with which the remainder of the structure was erected. The diameter of the well also precludes the possibility of its having been dug downward; it must have been gouged out from below, and the grotto was necessary to supply air to the thieves. It is inconceivable that the Pyramid builders would break one of their own ramp stones and leave its broken surface and a gaping hole in the side wall of their otherwise perfect gallery. If the well is a robbers' hole, it may explain why the Pyramid was empty when Caliph al Mamoun entered it and what happened to the missing coffer lid. A careful examination of the so-called unfinished subterranean chamber, which must have been the base of operations for the robbers, might disclose traces of their presence or show where they piled the rubble which must have accumulated as a result of their operations. While it is not entirely clear by what entrance the robbers reached the subterranean chamber, it is improbable that they used the descending passageway.