Wasn't the 'entrance-tunnel' created by arab looters?
The real main entrance was open during Graeco-Roman times, and tourists visited the pyramid even then. It was resealed late in the Roman period. Some centuries after the Moslems took over, the location of the real entrance was unknown, and one of the caliphs or poohbahs or whatever had a new one chipped into it. Once they found a passage, they followed it back down to the real entrance. It's possible that the so-called Robbers' Hole -- a sort of rabbit-warren-like passage -- was excavated at this time, but it could be ancient.