Pantheon

Rome's best-preserved ancient building, rebuilt by Hadrian around 126 AD over an earlier temple by Marcus Agrippa. Its concrete dome — 43.3 metres in diameter — remains the largest unreinforced concrete dome in the world after almost 1,900 years, and the open oculus at its centre lets natural light (and rain) fall directly onto the marble floor, where ancient drainage channels invisibly carry the water away. The building survived the fall of Rome because it was consecrated as a church in 609 AD, and it now holds the tombs of the painter Raphael and Italy's first kings.