The Great Pyramid of Cholula is said to be the largest pyramid constructed anywhere in the world.
However, it at first glance appears to be a hill with a church built on top.
The diorama below shows that it is 3 pyramids, one built on top of the other.
Beginning of
tunnel............................................midway................................light
at the end of the tunnel!
Inside the pyramid are some 5 miles of tunnels constructed by archaeologists. (A very interesting trek.)