The Pantheon is one of the most amazing buildings of Ancient Rome. It was built in about 120 AD and still stands today. It was built as a temple to all the gods, and was later converted into a Christian church. The most celebrated feature of the Pantheon is the mass concrete dome, which was the largest dome in the world for more than 1,000 years and is still the largest dome made of mass concrete. Only one stone dome was ever built bigger than it, the Duomo in Florence in about 1420. In the 20th Century, with the advent of metal-reinforced geodesic domes and tent-like structures, many domes have been built which are much larger, most notably the Millennium Dome in London, UK, but these are unlikely to have the permanence of the Pantheon.
The front of the Pantheon looks like a Greek temple with pillars and a triangular entablature on top. The entablature is inscribed M.AGRIPPA.L.F.COS.TERTIUM.FECIT which means 'Marcus Agrippa, Son of Lucius, third time Consul, made this'. There are three rows of eight pillars, each 14m high, which are topped with Corinthian capitals. Behind the pillars is the main body of the temple. There are two enormous bronze doors here.
Passing through the doors, we arrive at the main single room of the temple. This is a cylinder with a hemispherical dome on top. The dimensions of the cylinder are such that the height of the interior, from the floor to the top of dome, is the same as the width of the dome, 43.2m or 142ft. The centre of the dome has a giant circular opening, or oculus, 8.7m in diameter. This is uncovered, so that the building is open to the sky. Originally, the only lighting for the interior was the daylight which came through the opening. The inside of the dome is covered in a pattern of roughly square recesses. Originally, these were decorated with stars to give the impression of the sun (the opening) shining against a background of stars.
At ground level, there are seven alcoves around the walls of the main space, which originally held statues of the Roman gods. It is not known which gods were represented. They may have been the gods of the seven planets: the Sun, the Moon, Mars, Mercury, Jupiter, Venus and Saturn1. The floor is more or less flat, but dips slightly towards the centre so that any rain that comes in through the opening in the dome flows into the centre and drains away.
The original Pantheon, a small temple to all the gods, was built on the present site in 27 BC by Marcus Agrippa. This was replaced by the present building in about 120 AD. The present temple, despite the inscription, was not built by M Agrippa but was built by order of the Emperor Hadrian, around 117 - 125 AD. It was built as a temple to all the gods and the name Pantheon literally means 'Place for all the gods'.
The Pantheon was converted to a Christian church in 609 AD and still serves this function. It is officially named St Mary of the Martyrs. The church contains tombs of some famous people, including some kings and queens of Italy and the Renaissance painter Raphael.
The design of the vast concrete dome is ingenious. The dome varies in thickness, being 5.2m thick at the base, but narrowing to only 1.4m thick at the top. This helps to distribute the weight. In another clever weight reduction building technique, the builders mixed volcanic tufa and pumice into the concrete.
The cylindrical base wall of the temple is about 6m thick. The eight breaks in the wall for the recesses and the main doorway served a practical function in the building of the wall - they allowed the concrete to dry out. There are other hollows in the wall to serve the same function. They also reduce the weight of the wall so that the overall weight of the temple is not too great for the foundations. With a building of this size, there is a real danger of the whole thing sinking into the ground.
The pillared porch is independent of the main round room of the temple. It is not even connected to it, there being a very slight gap. This part has been criticised for being in a different, more Greek style to the rest of the building. This is not apparent, because due to buildings all around it, the round section is completely hidden from outside and can only be seen from inside. The Greek porch looks good from the outside and is an impressive entrance to an amazing building.