On entering the door, the effect you feel is meant to be overwhelming. You suddenly find yourself in this huge empty space which causes vertigo and makes you feel tiny. This is how you were supposed to feel in front of the gods and anyone going in today can have the same sensation as the Romans did almost 2000 years ago.
The space is a perfect sphere symbolising the vault of heaven; the height of the dome is the same as its diameter creating perfect balance and unique harmony; it is round so as to place all gods at the same level of importance.
Surrounding you, placed in seven splendid niches between two Corinthian columns the used to be the seven gods linked to the worship of planets, or considered to be such: the Sun, the Moon, Venus, Saturn, Jupiter, Mercury and Mars. With the advent of Christianity, some of them were used for small altars dedicated to Christian Martyrs.
The Pantheons greatness mainly comes from its mighty dome, still today the biggest brick dome ever built. Raising it using bricks alone would have been impossible; the ceiling would not have withstood the weight and would have collapsed.
As the Romans had no reinforced concrete they found another solution. This dome was built with a single casting of concrete in subsequent layers. The concrete was lightened by mixing it with lighter stones as it neared the highest point. Initially mixing the concrete with heavy travertine stone, going upwards using progressively lighter materials; like tuff stone. The top layer was made with the light-weight pumice stone.
At the centre of the dome, there is a 9 meter diameter hole, the Oculus. A brilliant idea. The Pantheon has no windows and the only light penetrates from above streaming down like a river of inner light; towards midday, the rays coming through the Oculus are particularly intense.
The Oculus was also to let the smoke from sacred fires out. The belief that the Oculus was built so that rain could not get in is not true. When it rains, it also rains in the Pantheon; the floor is slightly convex so the water flows away thanks to an effective drainage system.
Everything you see has not changed much in two thousand years. The columns, the marble, the inner decorations have not changed; even the floor is the same, built with precious marble from all over the Mediterranean Sea. Here walked emperors like Hadrian and Charles V.