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The origin of this Robbers' Tunnel is the subject of much scholarly discussion. According to tradition the opening was made around 820 AD by Caliph al-Ma'mun's workmen with a battering ram. The digging dislodged the stone in the ceiling of the Descending Passage which hid the entrance to the Ascending Passage, and the noise of that stone falling, then sliding down the Descending Passage alerted them to the need to turn left. Unable to remove these stones, the workmen tunneled upwards beside them through the softer limestone of the Pyramid until they reached the Ascending Passage.[136][137]
Due to a number of historical and archaeological discrepancies, many scholars (with Antoine de Sacy perhaps being the first) contend that this story is apocryphal. They argue that it is much more likely that the tunnel had been carved shortly after the pyramid was initially sealed. This tunnel, the scholars continue, was then resealed (likely during the Ramesside Restoration), and it was this plug that al-Ma'mun's ninth-century expedition cleared away. 