The Pantheon

The Roman Pantheon is one of the most extraordinary buildings that have survived from the ancient world. Its 142-foot diameter dome was built of solid concrete and remained the world’s longest-spanning dome until the 19th century. Its most ingenious engineering innovation was the use of different types of concrete aggregate at different locations throughout the building–heavy basalt where greater strength was required; lighter tufa and pumice where light weight mattered more than strength.