The Ancient Romans were skilled at concrete construction. When they built the Pantheon around 125 A.D., the skilled builders of Rome applied advanced engineering to the Greek classical orders. They gave their Pantheon massive 25-foot thick walls to support a huge dome made of solid concrete. As the height of the dome rises, the concrete was mixed with lighter and lighter stone material; the top is largely pumice. With a diameter that measures 43.4 meters, the dome of the Roman Pantheon ranks as the world's largest dome made of unreinforced solid concrete.
The ceiling of the Pantheon dome has five symmetrical rows of 28 coffers (sunken panels) and a round oculus (opening) at the center. Sunlight streaming through the oculus illuminates the Pantheon rotunda. The coffered ceiling and oculus were not only decorative, but lessened the weight load of the roof.
Learn More: The Pantheon by David Moore, P.E., 1995