The Pantheon is ancient Rome's best-preserved monument — a 2,000-year-old temple built by Emperor Hadrian and crowned by the largest unreinforced concrete dome in the world. Sunlight pours through the oculus, the 9-meter opening at its center, sweeping across the marble floor like a giant sundial. Once a temple to all the gods, it's now a church and the resting place of the Renaissance master Raphael and Italy's first kings. Step inside and stand where emperors, artists, and pilgrims have stood for two millennia.