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All my guidebooks tell me that the Pantheon is one of the key buildings of the ancient world.
Built more than 1800 years ago, the magnificent Pantheon building in Rome still stands as a reminder of the great Roman empire.
With its thick brick walls and large marble columns, the Pantheon makes an immediate impression on visitors. But for its time (AD 125) the most remarkable part of the building is the more than 43 metre high dome. It was the largest dome in the world until 1436 when the Florence Cathedral was constructed.
The huge concrete dome 143ft across with its central oculus open to the sky was a major engineering feat in its day.
At the top of the dome, the large opening, the oculus, is the only source of light.
The front portico has three rows of 8 columns, each one with a diameter of 1.5m. A huge bronze door gives access to the cylindrical building. Its diameter equals the interior height of 43.3m.
It became a Christian church in AD 608. Raphael and the first two kings of Modern Italy are buried here. 
Originally a temple for all pagan gods, the temple was converted into a church in 609AD. The Pantheon now contains the tombs of the famous artist Raphael and of several Italian Kings. Its ecclasiastic interior design contrast with the temple’s structural design, but the marble floor – which features a design consisting of a series of geometric patterns – is still the ancient Roman original.
Earlier Temples
Before the current Pantheon was built, two other buildings occupied the same site. The first one, a traditional rectilinear, T-shaped structure was built in 27 BC by the emperor Marcus Agrippa, son-in-law of the emperor Augustus. The temple was dedicated to the gods Mars and Venus. It burned down in AD 80 but was rebuilt by emperor Domitian. In AD 110 the building was struck by lightning and burned down again. In AD 118 emperor Hadrian commissioned for the Pantheon to be rebuilt but with a totally different, circular design. This time the Pantheon building would last much longer.
The Dome
The most important problem the Romans faced during the construction of the Pantheon was the massive weight of the large dome. In order to support it without proper reinforcement as it is common today, the thickness of the walls gradually decreased as the height increased. The Romans also used a different type of concrete for the dome as for the walls. At the base very thick (6m, 20ft) walls were constructed. At the top of the dome, a lighter type of concrete was used and near the oculus it is only 7.5 ft or 2.3 m thick. The use of coffers in the ceiling and the opening at the top also helped reduce the weight of the dome.
The huge, 60 tonne weighing columns used for the portico were quarried in Egypt. They were transported all the way to Rome using barges and vessels.

The columns support a pediment with an inscription attributing the Pantheon to Marcus Agrippa even though it was built by Hadrian.
The Pantheon borders the Piazza della Rotonda, a rectangular square with a central fountain and obelisk. The often crowded square is situated in the historic center of Rome, not far from the Piazza Navone, one of Rome’s most beautiful squares.