LLRX

The largest unreinforced concrete dome on Earth is 1,900 years old. There's no steel inside it. Nobody has built a bigger one since. The Pantheon, Rome. Rebuilt by Hadrian and finished around 125 AD. 43 metres across, more than twice as wide as any dome built before it. The trick is in the mix. Heavy stone at the bottom. Lighter stone as it climbs. Volcanic pumice at the top. The dome is over six metres thick at its base, but only about one metre at the top. And right in the middle, they left a hole. Nine metres wide, open to the sky. When it rains, it rains inside. The floor slopes gently, with holes to drain it away. The height to that hole and the width across are exactly the same. A perfect sphere would fit inside. Cassius Dio, writing some 75 years later, thought that was the point. Because of its vaulted roof, he said, it resembles the heavens. And over the door, Hadrian left someone else's name. The man who built the first temple here: M·AGRIPPA·L·F·COS·TERTIVM·FECIT Marcus Agrippa, son of Lucius, consul for the third time, built this. 🏛️