The Great Pyramid was open to tourists during later Hellenistic and throughout Roman times, then blocked back up during the 5th or 6th century, a job done so neatly that the later break-in during Muzzie times couldn’t find it. Anyway, there’s an oddball inscription carved on the lintel over the original entrance, it consists of four characters, hmm, there’s a topic about this somewhere...
http://www.freerepublic.com/focus/chat/1331709/posts?page=83#83
Also, parts of the original limestone facing on the Great Pyramid were carved with hieroglyphs, presumably dating back to the time of construction. That stone was pretty nice, and was carted off to build mosques and such in Cairo. The unfinished pyramid of Djedjefre — Khufu’s son and successor — at Abu Roash a few miles north of Giza was still being used as a quarry during the late 19th century.
The inside walls of the Great Pyramid were originally covered with plaster, a practice that was ubiquitous in ancient Egypt. Despite those centuries of Greek and Roman tourists, and the Muzzies, and moderns, some of that original plaster remained. A fragment of text on the plaster made reference to one year of Khufu’s reign, the Year of the Cattle Drive. It has since crumbled away.
Why do you think they blocked it back up again?