The best-preserved ancient building in the world, built by Emperor Hadrian around AD 125 on the site of an earlier temple. The concrete dome — 43.3 metres in diameter, identical to the height of the building — was the largest in the world for over 1,300 years and remains an engineering marvel. The oculus at its apex, 8.7 metres wide, is the building's only light source.
Arrive as early as possible — it fills fast and the interior is difficult to appreciate in a crowd. Raphael is buried here, as are two Italian kings. Now a church, so mass is celebrated on Sundays and at festival times.