As we have seen, before the Pantheon there was another, the first, built by Marcus Agrippa, the son-in-law of Augustus, and inaugurated between 27 and 24 BC.
From ancient sources we know that a century later, in 80 AD, that first building was destroyed by a fire that devastated much of the Campus Martius, and was restored by Domitian.
Recent excavations discovered the foundations of the oldest pronaos but no trace of structures of the time of Domitian, so it must be assumed that during his reign the building was repaired and put back into use in some way, but not rebuilt from scratch.
The first Pantheon had a circular plan, and even if it had a dome it must have been made of wood, which explains why it was destroyed a second time by a fire in 110 AD, apparently caused because the building was struck by a lightning.
Reconstruction work began immediately, because brick stamps from 113-114 AD were found in the outer walls; therefore the works started during the reign of Trajan.
To avoid the danger of fires, it was decided to build a masonry dome of exceptional size: even today it is the largest unreinforced concrete dome in the world, 43.3 meters in diameter.
It was an outstanding task, and in fact it is thought that the dome had been designed by Apollodorus of Damascus, Trajan's favorite architect. He was one of the greatest architects of antiquity and one of the few whose name is known to us because his achievements were exceptional.
The dome of the Pantheon is an absolute masterpiece of architecture of all time, both from an aesthetic and a structural and technical point of view. First of all the proportions: the diameter of the dome is equal to its height, which means that a perfect sphere was ideally contained inside the building.
From a technical point of view the secret was in the skillful use of building materials. In their lower part the walls are thicker and use robust and heavy materials such as peperino. Going up, increasingly lighter materials are used and the thickness of the walls is reduced, also thanks to the large coffers that lighten the weight of the vault.
You can discover these and other building secrets of the Pantheon in the book of Marina De Franceschini «Pantheon Architecture and Light» also in English edition.