PBS
Pbs: The Pantheon
This site from PBS lists the vital statistics of this ancient wonder as well as an explanation of how it was built. Also has a comparison between modern domed buildings and the Pantheon.
Other
Concrete and the Pantheon
Very detailed site explores the Pantheon with a scientific eye. It has many details about the process of building this wonder, the use of concrete, and why it has survived nearly 2000 years.