Pantheon
The Pantheon is the best-preserved ancient Roman building and an architectural marvel. Built by Emperor Hadrian around 126 AD, its unreinforced concrete dome (43.3m diameter) remained the world's largest for over 1,300 years. The oculus - a 9-meter opening at the dome's center - is the only light source and creates an almost mystical atmosphere. The building houses the tombs of Renaissance masters Raphael and the kings of unified Italy.