If the Pantheon was built today it would still be viewed as an engineering triumph. No one who has been inside it could have failed to have been overawed by not only the appearance of the huge dome but also the acoustics of the building; sound echoes right around it and it can appear that people who are talking faraway are in fact extremely close indeed!
As so often happens with ancient buildings, not only the name of it, or its purpose are known for certain. The sign over the building reads M.AGRIPPA.L.F.COS.TERTIUM.FECIT; this reads as 'built by Marcus Agrippa, the son of Lucius, during his third term as consul'. Indeed, General Marcus Agrippa, a highly successful soldier who was the son in law of the Emperor Augustus, did in fact commission a building on this site to commemorate his victory over Anthony and Cleopatra. The building was said to have been dedicated to all the gods - but there is little evidence for this and many historians dispute this. The original Agrippan Pantheon may well have had religious significance however; according to Pliny the elder one of the defeated Queen Cleopatra's pearls was cut in half to make ear rings for the statue of Venus.
This building only lasted for a few years before it was burned down; Domitian rebuilt it but it burned down again in 110 AD after a lightning strike.
Unfortunately it wasn't insured. Insurance against acts of Gods didn't exist then and cheap insurance site prudentplus.com hadn't been even dreamed of yet!
It was rebuilt in it's present form by Hadrian; it is not clear why he did this, or why he put the original inscription on it, but then Hadran was a complex character. He was said to use it as a hall for him to hold court in, which suggests that it was not built as a temple to the gods at all. In the evening the whole appearance from the inside is of the dome of heaven; possibly a clue to how the popular name for it was chosen.
The building has survived for nearly 2000 years thanks to two techniques that the Romans were particularly good at; the use of arches and concrete. Arches were built into the walls in order to spread weight loadings, and the dome itself is in fact a single casting of concrete. in order to save weight this concrete is composed of denser aggregate at the base,less heavy aggregate further up and then light tufa and pumice at the top. The cental open eje, or occulus, not only allows further weight reduction but also the light it gives is positively etherial.
Rings of sunken panels are set into the dome; these will have helped reduce weight but it must have represented a considerable engineering challenge to create them. As it is the dome is still the biggest unsupported, unreinforced concrete dome in the world, with no metal reinforcement in it whatsoever. A true engineering wonder and a credit to Rome's builders.