The Pantheon, Rome

Built on the site of an earlier temple commissioned by Marcus Agrippa during the reign of Augustus (27 BCE – 14 CE), it was ordered by the emperor Hadrian and completed in 126 CE.

Possibly the most interesting engineering feature, the unreinforced concrete dome survived the fall of an empire. Nearly 2,000 years later, nobody has ever built a bigger one: it still is the largest unreinforced concrete dome in the world.