Stumbling onto Piazza della Rotunda from the dark warren of streets surrounding it will likely leave you agape, marveling at one of ancient Rome’s great buildings—the only one that remains intact. The Pantheon (“Temple to All the Gods”) was originally built in 27 B.C. by Marcus Agrippa but was entirely reconstructed by Hadrian in the early 2nd century A.D. This remarkable building, 43m (142 ft.) wide and 43m (142 ft.) high (a perfect sphere resting in a cylinder) is among the architectural wonders of the world, even today. Hadrian himself is credited with the basic plan. There are no visible arches or vaults holding up the dome; instead they’re sunk into the concrete of the building’s walls. The ribbed dome outside is a series of almost weightless cantilevered bricks.

Animals were once sacrificed and burned in the center, with the smoke escaping through the only means of light, the oculus, an opening at the top 5.5m (18 ft.) in diameter. The interior was richly decorated, with white marble statues ringing the central space in its niches. Nowadays, apart from the jaw-dropping size of the space, the main items of interest are the tombs of two Italian kings (Vittorio Emanuele II and his successor, Umberto I) and artist Raphael (fans still bring him flowers), with its poignant epitaph. Since the 7th century, the Pantheon has been used as a Catholic church, Santa Maria ad Martyres, informally known as Santa Maria della Rotonda. The phone app “A Bit of Pantheon” offers helpful context for your visit.