There is also the problem of why Ma'mun was tunnelling inside the pyramid in the first place. Not only was the presence of the true entrance to the pyramid well known in classical times but people were also aware of the descending passage and the subterranean cavern at the very bottom of the pyramid. Strabo says of the original entrance to the Great pyramid: "The Great Pyramid, a little way up on one side, has a stone that may be taken out, which being raised up there is a sloping passage to the foundations."
Entrance door to the Great Pyramid
Strabo seems to be describing a door made of stone that is movable in some way, it can be moved upwards and outwards at the same time. This sounds like a hinged flap arrangement, with the hinge at the top of the stone. So was this description mere fantasy, or historical fact? We do not exactly know, but Strabo was obviously very familiar with the internal layout of the lower portions of the pyramid as he calls the lower cavern 'the foundations’, rather than using the more obvious term ‘chamber’. And Strabo is right, because the void below the pyramid is much more of a rough-hewn cavern than a smooth rectangular chamber.
Sir Flinders Petrie backed this quotation up with a detailed study of the entrances to the Vega (Bent) pyramid, the only pyramid that still has the doorways around the entrance intact. He found that on either side of the entrance, there were holes cut opposite each other, about 9cm in diameter by 14cm deep. These holes were just inside the entrance and only 15cm from the top of the passage. Petrie, not unreasonably, interpreted these as being the hinge sockets to swing the stone door from.
Behind these sockets, the passageway contained more door sockets. These were smaller vertical sockets, for a very lightweight door, perhaps made of wood and presumably to keep out the wind-blown sand. The diagrams below were developed by Petrie, based on his analysis of the Vega (Bent) pyramid entrance. The hinged stone door is clearly marked as the large shaded stone. It needs to be this shape, with a long top extending backwards, in order to counterbalance the weight of the stone. The amount of counterbalance at the top would have been judiciously arranged by the architect, so that the force required to open the stone was within normal human limitations, say about 25kg of force.
Great pyramid’s entrance closed and opened
Source: Ancient Origins