St. Peter's Basilica
No architect with any sense would try to design a dome without first checking out how these other two domes were made. Bramante checked them out. He discovered that the dome of the Pantheon, which had been standing for nearly 1500 years, was made of concrete. So that the concrete wouldn't be too heavy, it was mixed with pumice stone which comes out of a volcano and is full of gas holes so it is very light weight. Bramante learned how to make concrete like the Ancient Romans.