The Pantheon is a magnificent ancient temple in Rome that was later converted into the church of Santa Maria ad Martyres. Dating from 125 AD, this is the most complete ancient building in Rome and one of the city's most spectacular sights. http://www.sacred-destinations.com/italy/rome-pantheon

The Romans used a different type of concrete for the dome as for the walls. At the base very thick (6m, 20ft) walls were constructed. At the top of the dome, a lighter type of concrete was used and near the oculus it is only 7.5 ft or 2.3 m thick. The use of coffers in the ceiling and the opening at the top also helped reduce the weight of the dome.