The Pantheon, Rome — built by Emperor Hadrian around 125 AD and still in near-perfect condition. The oculus at the top of its dome is the building's only source of light and its only opening to the sky. The dome itself, cast from unreinforced concrete, remains the largest of its kind in the world.
© Livioandronico2013, CC BY-SA 4.0, via Wikimedia Commons
