Ancient Rome owed a great deal to its civil engineers. The remains of vast aqueducts, fortifications and arenas that helped make the city all powerful are images you tend to think of from that period in history.
But in the centre of the modern Italian capital is a Roman engineering marvel, virtually complete and still in use after nearly 1900 years.
The Pantheon is now a church but it was first constructed by Hadrian in 120 AD as a temple to the gods - hence its name. It replaced a earlier structure built by Marcus Agrippa 147 years earlier.
The rectangular portico outside is impressive enough, with an inscription commemorating Agrippa's original effort, providing an eerie link to the times of the Emperors. But in construction terms it is the interior that is awe inspiring.
A vast dome, 43.3m high, sits over a circular rotunda also 43.3m in diameter. Brick walls 6m thick create the drum supporting the unreinforced lightweight concrete dome. The cement was a mixture of lime, volcanic ash and crushed brick; the aggregate included tufa and pumice. Hollow, decorative coffers for the five horizontal courses making up the interior of the dome helped reduce its weight still further as did the circular opening at the apex.
Brick arches embedded in the structure of the wall act as internal buttresses, distributing the weight of the dome. The entire load is taken on a continuous concrete ring foundation 7.3m wide and 4.5m deep.
Sunlight pouring through the central opening in the cupola can be used to indicate precisely the time of day, and dates of the equinoxes and solstices.
Since Hadrian's day the monument has had a chequered history. Septimus Severus and Caracalla restored it in 202 AD. In 608 Pope Boniface IV transformed it into a church. Fifty five years later Byzantine Emperor Constans II stripped the gilded bronze tiles from the roof and Pope Gregory III replaced them with lead in 735. In Rome's years of decline in the Middle Ages the Pantheon was used as a fortress and poultry market. In 1632 Pope Urban VIII melted down the bronze from the portico for Bernini's baldacchino in St Peter's.
Now it is one of Rome's great sights and houses the tombs of Rafael and King Vittorio Emanuele II. But it is really a monument to the skills of engineers nearly 2000 years ago.