The Pantheon Dome, completed in 128 A.D., was one of the most impressive engineering accomplishments of its time -- indeed, for all time. Although Romans had been building small domed bathhouses and temples for centuries, no other dome would match the Pantheon Dome's 143-foot diameter for another 15 centuries.

The dome was constructed primarily of concrete, a material rarely used at that time for anything besides foundations. This allowed workers to more easily taper the thickness of the dome walls, which ranges from 5 feet near the top to more than 20 feet where the dome meets the building's vertical walls. Another impressive feature of the Pantheon Dome is the oculus, or "eye", which provides a 27-foot opening to the sky. The oculus is located 143 feet above the building's floor, making the dome as high as it is wide.

The Pantheon's completion was certainly a crowning achievement for its designer, Hadrian, the 13th emperor of Rome. It did, however, present its share of engineering problems. Most important of these was that the dome required significant lateral support in the form of enormous concrete "step rings" near where the elegantly arching dome meets the vertical walls of the lower part of the building.

Throughout most of a dome, inward and upward forces counteract the force of gravity -- the weight of the dome itself. These opposing forces cause the individual sections of the dome to push hard against, or compress, one another. Where compression is high, the dome is stable. At the lower sections, where the dome meets the building's vertical walls, however, the necessary compressive force is missing. To save the dome from collapse, Pantheon engineers placed the massive concrete step rings around the base of the dome. These rings are heavy enough to resist the outward force of the dome and have kept the dome stable and relatively free from cracks for nearly 2,000 years.