The Romans were masters of concrete, using pozzolana, a volcanic ash, instead of portland cement as we use today. The Pantheon in Rome is one of the greatest demonstrations of their skill; the great dome exactly encloses a 43.3 meter (142 feet) sphere. Built in the year 126, it remains the world's largest unreinforced concrete dome.
The concrete mix changes as the dome gets higher, with pumice and other light volcanic rocks mixed in to reduce the weight. The oculus, or hole in the ceiling, is the only source of light; the floor slopes to a drain, catching the water when it rains.
I almost wept at its beauty and wonder.