Original entrance of the Great Pyramid. When it was first built, this pyramid was covered on all four sides by smooth casing stones of polished limestone. These were joined together so accurately that the gap between the joints averaged 1/50th of an inch. This original entrance was never discovered from the outside - the caliph Al Mamoon was the first person to enter the pyramid in 820 AD, which he did by forcing a way through the solid limestone core blocks until he hit the descending passage. He then followed this back to the outside of the pyramid which revealed the original entrance, concealed by a polished limestone casing block, pivotted to swing open. The Great Pyramid lost most of its casing stones in a major earthquake which shook the area in 1301 AD.