<P> Although succeeding pyramids were smaller, pyramid - building continued until the end of the Middle Kingdom . However, as authors Brier and Hobbs claim, "all the pyramids were robbed" by the New Kingdom, when the construction of royal tombs in a desert valley, now known as the Valley of the Kings, began . Joyce Tyldesley states that the Great Pyramid itself "is known to have been opened and emptied by the Middle Kingdom", before the Arab caliph Abdullah al - Mamun entered the pyramid around AD 820 . </P> <P> 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 . </P> <P> He also discusses a story told by Herodotus . Herodotus visited Egypt in the 5th century BC and recounts a story that he was told concerning vaults under the pyramid built on an island where the body of Cheops lies . Edwards notes that the pyramid had "almost certainly been opened and its contents plundered long before the time of Herodotus" and that it might have been closed again during the Twenty - sixth Dynasty of Egypt when other monuments were restored . He suggests that the story told to Herodotus could have been the result of almost two centuries of telling and retelling by Pyramid guides . </P>

When was the great pyramid of giza discovered