The Pantheon is one of the best preserved buildings of all acient Roman structures, which was constructed almost two thousand years years ago, and it still remains the largest unreinforced concrete dome in the world. the entire building is made up of around 4,535 metric tons of concrete, which was built without the help of cranes, due to the fact there weren't such things back in the day.
For further reading of this magnificent building go to http://www.romanconcrete.com/docs/chapt01/chapt01.htm, which is a indepth break down of The Pantheon by David Moore.