Rome's Pantheon, finished around 125 AD, still holds the record for the world's largest unreinforced concrete dome, nearly 1,900 years later.
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What we found
The Pantheon's enormous unreinforced concrete dome manages its weight with a thinning shell, lighter aggregate higher up and a massive supporting structure. The oculus is open to the sky. The engineering is remarkable without calling it impossible.
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Transcript
This Roman concrete dome spans a hundred and forty-two feet. There is no steel skeleton inside. So how does all that weight stay above your head? The Pantheon in Rome is nearly nineteen hundred years old. Start at the bottom. Thick walls support the dome around its edge. Then look up. The concrete shell gets thinner as it rises. The builders also changed the material inside it, using lighter volcanic stone higher up. And that bright circle is not a skylight. There is no glass. The oculus is a real opening, about thirty feet wide. Rain comes in. The floor below has drains. Almost two thousand years later, you can still stand underneath the result.
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