Pantheon

Nearly two thousand years after it was built, the Pantheon's dome remains the largest unreinforced concrete dome in the world. Its interior height matches its diameter of about 43 metres, and the only light enters through the open oculus at the top, which sends a moving disc of sunlight across the coffered ceiling and lets rain fall onto the marble floor below.

The present building dates from the reign of Hadrian in the early second century, although the inscription on the portico credits Marcus Agrippa, who built an earlier temple on the site. Visitors pass between massive granite columns and through ancient bronze doors into the rotunda. In 609 it was consecrated as a church, Santa Maria ad Martyres, and that conversion explains why it survives so intact. It still holds Mass today.

Inside are the tomb of Raphael and the burials of Italy's first kings, Vittorio Emanuele II and Umberto I. A visit is usually brief, and an hour or less covers the building.