Pantheon, Rome

The Pantheon, completed around 125 AD under Emperor Hadrian, has the world's largest unreinforced concrete dome at 43.3 metres in diameter — a record it has held for nearly 1,900 years. The oculus at its apex, 8.8 metres across, is the building's only light source and creates a beam of light that moves across the interior as the sun moves, a deliberate solar clock.