It was thought that this inscription referred to the present temple, but Agrippa's temple was destroyed and then restored several times by Domitian and Trajan. It was fully reconstructed by Hadrian around 125 A.D., but he preserved Agrippa's inscription. This new Pantheon has been attributed to the architect Apollodorus of Damascus, one of the most famous architects of the Ancient world. The Greek influences we can appreciate in the portico (very similar to the pronaos of the Greek temples) are due to the great admiration Hadrian felt for Greek culture. After the portico, we find a big circular cella or rotunda, covered with a semispherical dome with an oculus opened to the sky. The portico and the rotunda are linked by a polygonal structure. The dome rests on a cylindrical drum and it is decorated with coffers, which were designed to reduce the weight of the dome; it is the world's largest concrete dome. The walls of the temple have the same height as the radius of the dome and the total height is equal to the diameter, so a perfect imaginary sphere can be inscribed in the interior of the Pantheon. The structure of the Pantheon is very different from the conventional structure of the roman temples, which was very influenced by the preceding Etruscan and Greek models, so when it was built it represented an important innovation. Although we can find examples of domes resting in a cylindrical drum in some previous buildings, like the thermal baths of Agrippa or Caracalla, the dome of the Pantheon is the first one built with such big dimensions.
The materials used in the Pantheon were also very innovative; the concrete applied to the construction of the dome and the marble, used in the floor, which was a symbol of magnificence and splendor.
Agrippa's temple was smaller than the present temple and it was facing to the south (the actual Pantheon is facing to the north). It seems that the structure was very similar to the actual temple, and it was based in the architectural principles of Vitruvius. In the pronaos, now decorated with Corinthian columns, were erected the statues of Augusto and Agrippa as the promoters of the temple, so what people saw at first sight were the figures of the Emperors, as the developers of this magnificent architectural work.
The geometry of the Pantheon has a deep symbolic meaning. According to the plane geometry and Vitruvius' ideas, the squared Greek portico represents the Earth and the circular rotunda the Heaven. The rotunda and the dome are linked by an octagon, by which the squaring of the circle is achieved; this will mean the indestructible union between matter and spirit. In the floor we can also find geometrical drawings of circles inscribed inside squares. Furthermore, the lower part of the drum's wall is drilled with niches, suggesting a continuity in the unlimited space. The columns that support these niches have been constructed as if they were not supporting the dome, so it seems to be floating in the air. The dome represents the vault of heaven and each of the five levels of coffers represents the five concentric spheres of the Ancient planetary system.
The central oculus, which illuminates the interior of the temple, symbolizes the