The Pantheon is a remarkable building. It was completed by the Emperor Hadrian around 126AD, and has been in continues use ever since.
The dome starts at about 6m thick, and narrow to about 1.5m at the top (that’s 4535 metric tons). The dome is 43m in diameter and staggering to look up at. It is a remarkable piece of engineering, especially considering that it was build almost 2000 years ago.
PS: It’s free to visit.