Let's imagine you're standing on a bowling green, bowling ball in hand. You want to roll the ball along the green to its target, be it the jack or some skittles, but you're not allowed to push or help it along its way. All you have is gravity. The surely non-regulation bowling green must therefore be set on a slope. Given that a green is typically less than 40 metres long, you don't need to be standing too much higher than your target as long as the slope is even, but imagine now that this bowling green stretches for 20 km. Yes, definitely non-regulation. And imagine that the spot you're standing on is only 17 metres higher than your target, but in between are hills and rivers and valleys and forests. 17 metres difference in 20 km - the equivalent for a 40 metre bowling green would allow you a total height difference of just 3 cm. No giving the ball a helping hand, it just has to roll continuously. This was the problem the Romans faced 2000 years ago when building an aqueduct to serve the city of Nimes, and this was why they built the Pont du Gard.
Ok, the Romans weren't thinking of bowling when they built their aqueduct, but the core issue was the same: gravity. The Romans loved water, and in all their big cities went to great efforts to ensure they had a water supply, for drinking and for their public baths and fountains. Rome itself had eleven aqueducts. A river running through a city certainly had many uses, but without the use of modern technology and motorised pumps there was no way to get the water to go upwards; besides, the rivers were often polluted. Therefore, to get a source of clean running water the Romans had to find a source of water that was higher than the city, usually from nearby hills. They would then build an aqueduct to connect this source to the city: the water would simply follow gravity. Ultimately it would flow along the channel and end up into what was called the castellum divisorium, a wide basin that had pipes leading from it and supplying the city. Often, all this was a fairly routine task of engineering. In the case of Nimes it was anything but.
In the 1st Century AD, the Romans embarked on an extensive building program for the city of Nimes, giving it city walls, temples, and amphitheatres, much of which can still be seen today. Naturally, public baths were required also. The only viable source of water was 20 km away, at a spot a mere 17 metres higher than the city. In case this seemed impossible enough, the land in between made a a straight-line aqueduct impossible. After considerable planning, it turned out that the only possible route would take a circuitous 50 km path. Only a 17 metre drop across 50 km - you try getting a bowling ball to do that. To get some comprehension of quite how gradual a drop that is, if you're currently in a room that's five metres long (a normal living room size), then that's a drop of less than 2 mm. While maintaining this gradient, tunnels had to be dug, a lake was drained, long routes around hills found, and of course, bridges built. Building such an aqueduct in rough hilly forested terrain without modern equipment to such precision has been called one of the greatest ever feats of Roman engineering. That it also happened to include building the biggest bridge then known was just a bonus.
Although the Pont du Gard is often called an aqueduct, that description is a little misleading - it is a bridge that is part of an aqueduct. And it's not the only bridge of Nimes aqueduct, there are many more along the 50 km stretch. But it is certainly the biggest and most spectacular. Further down was once a 100-metre long and 25-metre high bridge, now ruined. Others still stand at a still fairly hefty 8 metres tall. But the Pont du Gard is by far the biggest, measuring 275 metres in length at the top, and just under 49 metres high. Regarded as an architectural masterpiece, it is indeed Roman engineering at its finest: a series of arches, massive, extremely well-built, and attractive. In fact, it is just about as big as the Romans were willing to risk. Other aqueducts had river gorges deeper than 50 metres, but the Romans weren't comfortable with building bridges this high. For these they used siphons - kind of like long U-shaped lead pipes, where the exit end was slightly lower than the entrance. But these were expensive, and where possible the Romans preferred to build bridges, which looked far grander visually. The Romans weren't averse to showing off a little.
In effect, the Pont du Gard is three bridges built on top of each other. Two large arched bridges and a smaller multi-arched top section which contains the water channel, sloped ever so slightly to allow the flow of water - 2.5 cm difference over the 275 metre length. Built from around 50,000 tons of local limestone blocks, the use of mortar was mostly avoided by having the blocks cut very precisely. This 2000-year-old bridge stays up simply by virtue of being heavy and a snug fit.
Did it do its job? Yes, very well. The fact it still stands today is testament to the Roman expertise in construction. Although, post-Roman, it was saved from being willfully dismantled because it doubled as a handy footbridge, it wasn't exactly kept in good shape. In the 16th Century, in what can only be described as sheer lunacy, the middle section piers (that is, the legs of the arches) were cut away by half on the upstream side so that carriages could fit by. This is like taking an axe to a tree and chopping until reaching near the centre. Remarkably, it didn't collapse, or indeed suffer any ill effect. That damage was subsequently repaired, but in the 19th Century a road bridge was built right next to it. This was a public road with a lot of cars driving across, right up until the 1990s before the authorities finally saw sense - with the help of UNESCO - and pedestrianised it.
The Pont du Gard has also survived centuries of flooding, flooding which as recently as 1958 saw other, modern, bridges swept away, and the river level right up to the second tier. So what I'm saying is that the Pont du Gard is solid. It is Roman engineering at its best. How many modern buildings could withstand 2000 years of neglect and occasional abuse?
As a functional water channel for Nimes aqueduct, its lifespan wasn't long. This was no reflection upon the Pont du Gard, more that of a slowly crumbling Roman empire. By the 3rd Century, the area was in decline, and by the 4th Century the Visigoth tribes from northern Europe were occupying the area - and they didn't care for sophisticated bathing quite in the same way as the Romans. The 50 km aqueduct system needed maintenance, and without it calcium deposits from the water eventually built up in the system and reduced then halted the flow. By this time damage to other parts of the aqueduct would have ended the water supply anyway. The centuries rolled on, with plenty other wars and conquests and a whole lot of destruction, but the Pont du Gard managed to fall under the radar, not having political or religious associations. It was just a bridge, and not a particularly strategic one at that, so everyone left it alone. The Pont du Gard survived the many generations and ages by being big and strong - and entirely non-threatening.
These days, the Pont du Gard is one of France's most visited attractions, a UNESCO World Heritage Site, and shares an unofficial honour with the Millau Viaduct - appearing on money. The Millau Viaduct in all but name appears on the €500 note, and though again not formally named, the Pont du Gard appears on the €5.
The Pont du Gard was a very late addition on my list, just sneaking on while on holiday in the south of France last year. I'd previously considered it, but had dismissed it as just a big Roman bridge. But I may as well dismiss the Millau Viaduct as just a big modern bridge, or the Great Wall as just a very long wall. The Pont du Gard represents just about the best of Roman engineering - an immense technical achievement of size and subtlety, that looks great. Other than the word "wow", there can be no simpler way to describe a potential Wonder. So let's see if it can wow me.
I'll be visiting the Pont du Gard next summer, and will give a fuller account of it and its history then, as well as my own impressions.