The Pantheon has been in use for almost 1,900 years, and its concrete dome is still the largest unreinforced one in the world. Here's what you're looking at, and what to listen for when you step inside.
The inscription says Marcus Agrippa built it. He did build the first temple here, around 25 BC, but that one burned. Hadrian rebuilt it completely and kept Agrippa's name on the front, a rare piece of modesty for an emperor.
The builders mixed heavy stone into the concrete at the base and light volcanic pumice near the top, and cut the coffers to remove more weight. That's how a dome as wide as it is tall has stood for nineteen centuries without steel.
The oculus is open to the sky, so rain falls into the building. The marble floor is very slightly domed, and 22 small holes drain the water away.
The building you see was completed under Emperor Hadrian, around AD 125, on the site of an earlier temple built by Agrippa.
The oculus is the building's only source of natural light and lightens the top of the dome. Rain falls through it and drains through small holes in the floor.
The painter Raphael, and two kings of Italy, Victor Emmanuel II and Umberto I, along with Queen Margherita.