Of all ancient Rome’s great buildings, only the Pantheon (“Temple to All the Gods”) remains intact. It was originally built in wood in 27 b.c. by Marcus Agrippa but was entirely reconstructed by Hadrian in the early a.d. 2nd century after it was destroyed in a fire. This remarkable building—once entirely covered in white marble, 43m (142 ft.) wide and 43m (142 ft.) high (a perfect sphere resting in a cylinder), and laced with white marble statues of Roman gods in its niches—is among the architectural wonders of the world, even today. Hadrian himself is credited with the basic plan, an architectural design that was unique for the time. There are no visible arches or vaults holding up the dome; instead they’re sunk into the concrete of the walls of the building, while the ribbed dome outside is a series of almost weightless cantilevered bricks. Animals were once sacrificed and burned in the center, and the smoke escaped through the only means of light, the oculus, an opening at the top 5.5m (18 ft.) in diameter.

The interior now houses the tombs of two Italian kings (Vittorio Emanuele II and his successor, Umberto I), and the resting place of Raphael (fans still bring him flowers), between the second and third chapel on the left. The Pantheon has been used as a Catholic church since the 7th century, the Santa Maria ad Martyres, but informally known as “Santa Maria della Rotonda.”