What Roman structure has the world's largest unreinforced concrete dome? The Pantheon.
The Pantheon in Rome has the world's largest unreinforced concrete dome. Its interior dome spans about 43.3 metres, and its height from floor to oculus is roughly equal to its diameter, creating a remarkable spherical geometry inside the rotunda.
The present building was probably commissioned by Emperor Hadrian around AD 126 on the site of an earlier temple associated with Marcus Agrippa. Its massive dome rests on a thick brick-faced concrete rotunda. Roman builders reduced the load with progressively lighter aggregate: heavy materials such as travertine were used lower down, while tufa and pumice appear nearer the top.
The central oculus, about 9 metres across, is the dome's only opening. It supplies light and ventilation, while drains in the floor handle rainwater. Coffered panels also reduce weight and create the dome's distinctive appearance. The Pantheon is often confused with the Colosseum, but the Colosseum is an amphitheater; the Pantheon's defining achievement is its immense concrete rotunda and dome.