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The hole in the roof isn't a mistake. It is the only reason the building is still standing.
When people walk into the Pantheon, they look up at the rain falling through the 9-meter opening and ask: "Did they run out of money? Why didn't they finish the roof?"
The answer is Roman genius.
Why is the hole there? (The Engineering) If the Romans had closed the dome with heavy concrete, the weight at the top would have been too crushing. The dome would have collapsed under its own stress 2,000 years ago. The Oculus (the eye) acts as a "Reverse Keystone." It actually relieves the structural tension. It lightens the load at the weakest point of the dome.
The Secret Recipe (Why it doesn't collapse) The Romans didn't just pour one type of concrete. They were the masters of chemistry.
At the bottom (the base): They used concrete mixed with heavy Travertine rock for strength.
In the middle: They switched to lighter Tuff rock.
At the very top (near the hole): They mixed the concrete with Pumice (volcanic rock so light it floats on water).
The top of the dome is incredibly light. If they had used the heavy bottom concrete at the top, the Pantheon would be a pile of rubble today.
Why doesn't it flood? It has rained inside the Pantheon for nearly 2,000 years. So why isn't the floor a swimming pool? If you look closely at the marble floor, it isn't flat. It is slightly convex (curved in the center). This guides the rainwater toward 22 tiny, hidden drainage holes cut directly into the marble. The water flows into an ancient Roman sewer system underneath the building—a system that still works today.
The "Sun" Dial The hole wasn't just for weight; it was for the gods. The Pantheon was a temple to "All Gods." The Oculus allowed the heavens to enter the temple. On April 21st (the birthday of Rome), the sun strikes the entrance grill perfectly at noon. It wasn't just a building; it was a functioning astronomical clock.
So no, they didn't forget the glass. They built a machine made of stone that has survived Barbarians, Popes, and gravity for 19 centuries.
