The Pantheon was built in 27 B.C. by Agrippa and re-built by Adriano. At first it was a place for pagan worship. Later (in the 7th century) it was transformed into a Christian basilica called St. Maria della Rotonda. This change allowed it to remain whole after the spoilings which were made by the Popes for the buildings in classical Rome. Then it became the mausoleum of the Italian kings Vittorio Emanuele II and Umberto I.
Since its construction (which happened almost two millennia ago), its dome has been the biggest in the world. It is made of non-reinforced concrete.
The hole in the dome
Look up when you enter; this is the first thing to do when you step inside the Pantheon. A tilted ray of sun shines from the “oculus”, the round hole of 9 metres at the top of the dome which lights up the whole building. As soon as you are inside, you will discover the curious round structure and the porch made of Corinthian columns.
The rain in Pantheon
Almost everyone, both Romans and tourists, asks if rain enters the building. Here you have the answer: The ascensional current coming from the hole destroys the drops. This way, when it rains slightly, the quantity of water which enters the building is limited. If the rain is hard, it will always rain less inside than it will outside.
Moreover, both the central and lateral holes on the floor drain the water; for this reason, you will not see puddles on the floor.