The Pantheon in Ancient Rome: how it worked

History What it was

The Pantheon was a temple in the center of Rome. Its name comes from Greek words meaning "all the gods". The building standing today was begun under the emperor Trajan and completed under Hadrian in the early 2nd century AD. The inscription on the front names Marcus Agrippa, who built an earlier temple on the same spot more than a century before.

The Pantheon's columned porch. The inscription names Marcus Agrippa. Photo: daryl_mitchell from Saskatoon, Saskatchewan, Canada, CC BY-SA 2.0, via Wikimedia Commons

History How it worked

The Pantheon has a traditional columned porch at the front, but behind it is a huge round hall covered by a dome. The dome is about 43 meters across, the same as the height from the floor to the top, so a giant ball could fit perfectly inside.

The dome is made of Roman concrete. Builders used heavier stone in the concrete near the bottom and lighter volcanic stone near the top to reduce the weight. Sunken square panels, called coffers, cut into the inside of the dome also make it lighter.

At the top is the oculus ("eye"), an open circle about 9 meters wide. It is the building's main source of direct light, and a circle of sunlight moves around the walls as the day passes. When it rains, water falls in and drains away through small holes in the gently sloping floor.

A beam of sunlight through the oculus moves across the inside of the dome. Photo: NormanB, CC BY-SA 3.0, via Wikimedia Commons

History Why it survived

In AD 609 the Pantheon was turned into a church, which meant it was cared for when many other ancient buildings were taken apart for stone. Nearly 1,900 years after it was built, it remains one of the largest unreinforced concrete domes in the world.