The Pantheon is a temple completed in 25 BCE by Agrippa. It was severely damaged by fire late in the first century CE. The Rotunda was erected by the Emperor Hadrian in CE 120-4. The Rotunda is circular, with the same internal diameter and height of 43.43 m (142 ft 6 ins). A massive circular foundation supports the walls of brick-faced concrete, which was not solid but comprised an elaborate constructive system.
-- information from: Palmes, J. C. (1975). A History of Architecture (18th Edition). New York: Charles Scribner's Sons.
Vickers, G. (1999). Key moments in architecture: the relationship between man, buildings, and urban growth as seen in the metropolis through the ages. New York: Da Capo Press. p. 44.