The opening at the top was the original entrance to the Pyramid that was only discovered again in 820AD. The opening that visitors use to gain entry is below where you see the group of people milling about.

At the bottom, near the child in the yellow T-shirt, you can see a simulated inclined wall of white limestone. This is how the outer covering or casing of the pyramid would have looked at one time. It would have been a dazzling site. Then next largest pyramid on the site still has some of its casing intact.