[Ezekiel]

After the Great Pyramid was initially sealed, it's original entrance was hidden and faced with smooth limestone. Because this blended in so well with the surrounding casing, the opening was invisible. Around 820 AD, Abdullah Al Mamun mobilized men to bore a tunnel into the pyramid to search for chambers and treasure. Due to the difficulty of the task of breaking up the hard rock, fires were built to heat the rock and then cold vinegar was poured over the heated rock. Battering rams were used to pound away the weakened rock and clear a tunnel. Eventually, a passageway was found which descended into the lowest chamber of the pyramid. Following this passageway back upward, the original entrance was finally located. In these pictures of the NORTH side you can see the intrusive entrance lower down, and the original entrance higher up flanked by angled stones:

http://guardians.net/egypt/gp1.htm

[SunkenCiv]

The Great Pyramid was opened during the Late Kingdom (or perhaps a little before, during the Persian occupation), and like other tombs and such in Egypt, was quite the tourist attraction. Sometime during the century or so preceding the Islamic conquest of Egypt, the local authority (I believe it was the Byzantine Empire) closed the thing back up. During the few centuries it was open, someone managed to cut four characters in an alphabetic script, they can be seen over the entrance, and are in a so-far-unknown language.

As you noted, the muzzie bozos couldn’t find the original entrance that had not been any great challenge to the earlier authorities.