[202]

Looting
Authors Bob Brier and Hoyt Hobbs claim that "all the pyramids were robbed" by the New Kingdom, when the construction of royal tombs in the Valley of the Kings began.[203][204] Joyce Tyldesley states that the Great Pyramid itself "is known to have been opened and emptied by the Middle Kingdom", before the Arab caliph Al-Ma'mun entered the pyramid around 820 AD.[136]
I. E. S. Edwards discusses Strabo's mention that the pyramid "a little way up one side has a stone that may be taken out, which being raised up there is a sloping passage to the foundations". Edwards suggested that the pyramid was entered by robbers after the end of the Old Kingdom and sealed and then reopened more than once until Strabo's door was added. He adds: "If this highly speculative surmise be correct, it is also necessary to assume either that the existence of the door was forgotten or that the entrance was again blocked with facing stones", in order to explain why al-Ma'mun could not find the entrance.