The Pantheon, a 1,900-year-old Roman temple containing the world's largest free-standing dome, is made primarily of concrete, volcanic rock and granite. The most remarkable part, the rotunda and dome, is made of concrete that includes lightweight pumice as part of the aggregate.
The Pantheon was built to honor all of the Roman gods. The structure's dome is 43 meters high, 43 meters in diameter and free-standing. The dome is attached to a granite portico, or entrance porch. The 39-foot-high gray granite portico columns, each weighing 60 tons, came from a quarry in Egypt. The stone in the rotunda is primarily volcanic or shaped concrete, honeycombed with hidden arches to add strength to the entire structure.
The dome is heavier and thicker at the bottom than at the top, and the center of the dome has an 8-meter-wide opening called an oculus. Its structure is further lightened at the top by using more lightweight volcanic rock in the concrete aggregate, with heavier granite-based concrete aggregate used toward the dome's base. This is a construction technique that is still used today. Arches are used throughout the structure to add strength without adding too much weight to the foundation.
Today, nearly 2,000 years after its construction, the Pantheon still shows no signs of settling or cracking. It is still in use as a Roman Catholic church, the Santa Maria Rotonda.