How has the Pantheon's concrete dome survived for nearly 2,000 years?

The Pantheon's dome has survived because the Romans designed it to be as light as possible exactly where weight is most dangerous, and because people never stopped using the building. Almost two thousand years after it was finished, around AD 126, it is still the largest unreinforced concrete dome in the world.

The trick is in the recipe. At the base, the dome is 6.4 metres thick and made with heavy travertine stone mixed into the concrete. Higher up, the builders switched to terracotta tiles, and at the top to light, porous tufa and pumice, while the shell thins to just 1.2 metres. Then they left the very top out altogether: the famous round opening, the oculus, removes weight where the dome would be weakest. Engineers estimate that with ordinary concrete throughout, the stresses would be about 80% higher.

The concrete itself may help too. In 2023, researchers at MIT found that the little white lumps of lime in Roman concrete, long blamed on sloppy mixing, can react with water seeping into a crack and seal it with fresh calcium carbonate.

Finally, the Pantheon was lucky. In 609 it became a church, and it has been in use ever since, which is a large part of why it is one of the best-preserved Roman buildings. Rain still falls through the oculus, onto a gently sloping floor with drains.