The Pantheon — Greek for “a temple to all gods" — is a former Roman temple constructed in Rome, Italy, from 113 to 126 A.D. At almost 2,000 years old, it is the oldest, continuously used structure in history. The key to its longevity is a concrete hemispherical dome roof. At 142 feet (43 m) diameter, it was and is the largest unreinforced concrete dome in the world. … read more