“... my own opinion is that the [Pantheon's] name is due to its round shape, like the sky."
Cassius Dio, History of Rome (c. 220)
Domes, like arches, present problems for architects and engineers: They remain unstable until the final stone is put in place and have to support their own weight without collapsing. It was the Romans who in circa 100 first solved these technical problems when they managed to build a true dome, that is, an unsupported half-sphere.
The greatest of these is the Pantheon in Rome, which was erected in circa 123. Roman engineers tackled the problem as if it was a series of circular barrel vaults, or arches, arranged in a circle across a central point, and used concrete as a building material, in this case a mixture of lime, pumice, pieces of rock, and volcanic ash. A template for each arch was erected
on scaffolding and served as the mold for the poured concrete. The dome was built up in sections, with heavier concrete being used at the thicker base and lighter concrete toward the thinner top. The weight of the dome is concentrated on a ring at the top of the structure surrounding an opening that lets in the light, and reduces the weight at the top. The result is to push the weight of the dome toward its base and down to the floor below. Once the dome was completed and the concrete dry, the supports could be removed.
