Pontcysyllte Aqueduct

Listen first. Far below your feet, the River Dee runs over stones, a low and steady sound that rises up the valley. Up here, the only sound is the soft slap of canal water against iron. The air moves freely, because there is almost nothing between you and the sky. On one side, a railing. On the other... just the edge of a metal trough, and then a long drop into green.

This is the Pontcysyllte Aqueduct, a few kilometres east of Llangollen in north-east Wales. It carries a canal across the Dee valley, and it carries it very high. The deck stands roughly thirty-eight metres above the river, and the whole crossing runs a little over three hundred metres. It is the longest and highest aqueduct in Britain. It is also often described as the highest navigable aqueduct in the world. Boats float here, in mid-air.

The name sounds hard to pronounce, so here is a gentle guide. Say it as something like 'pont-kuh-suth-teh'. The Welsh 'll' is a soft, breathy sound, made with the tongue against the roof of the mouth. Locals will not mind if you struggle. Most visitors simply call it 'the aqueduct', and everyone knows which one you mean. The name itself comes from an older bridge site nearby, not from the canal.

The story begins in the 1790s, in the great age of British canal building. Coal, iron and limestone sat in these hills, and industry needed a cheap way to move them. A new waterway, the Ellesmere Canal, was planned to link rivers and markets across the border country. Work on the aqueduct started in 1795. It opened in 1805, after roughly ten years of labour. Today this stretch of water is known as the Llangollen Canal.

Two engineers stand behind the design. Thomas Telford, a Scottish stonemason who became one of the greatest civil engineers of his era, led the work on site. William Jessop, an older and highly experienced canal engineer, was senior on the project. Historians still discuss how much credit belongs to each man. Telford's name is the one most visitors hear, but the aqueduct was a shared effort, and a bold one for both of them.

Here is the idea that made it possible. Older aqueducts carried water in a channel of stone and clay, which was extremely heavy. A structure that heavy could never stand this tall. So the builders used cast iron instead. The canal runs through a long iron trough, made of plates bolted together. Iron is strong, and the trough is far lighter than a stone channel. That lightness let the piers rise so slender and so high.

Look down at those piers, if you can bear the height. There are eighteen of them, made of local stone, and they carry nineteen iron arches. Here is a detail most people miss. The upper parts of the piers are hollow. The builders left them hollow to save weight and stone. From a distance they look solid and heavy. In fact, they are cleverly light, which is why the whole line looks so thin against the sky.

The iron came from close by. It was cast by the ironmaster William Hazledine at a foundry near Cefn Mawr, the village on the hill just to the south. The trough plates have a slight wedge shape, a little like the stones in a classic arch. Each plate pushes against its neighbour, so the load spreads along the curve. Two centuries later, that same iron still holds the water you see beside you.

And how do you stop an iron box from leaking? Accounts from the period mention joints sealed with Welsh flannel and lead. Another story is more colourful. Locals say the mortar in the piers was mixed with ox blood, along with lime and water. Treat that one as a popular tale rather than proven fact. Whatever the recipe, the result has lasted. Narrowboats still cross here every day in season.

Now look at the two sides of the trough. The towpath runs along one side, with a railing for walkers. It was built for the horses that once pulled the boats. On the other side, there is no towpath at all. The iron wall rises only a short way above the water. If you watch a narrowboat pass, notice the person at the tiller. On that open side, they look straight down into the valley.

In 2009, the aqueduct and about eighteen kilometres of the surrounding canal became a UNESCO World Heritage Site. UNESCO is the United Nations body that lists places of value to all humanity. The listing honours more than one structure. It covers the whole route, including the Chirk Aqueduct and tunnels further south. Together, they show how engineers at the start of the industrial age pushed canals through steep and difficult land.

At the northern end lies Trevor Basin, a small harbour of canal water. Boats wait there, people buy ice creams, and families gather before they cross. To the south, a footpath drops down into the valley. Walk it, if you have time and steady legs. From the riverbank, the aqueduct looks completely different. You see the arches against the sky, and boats sliding along the top, small as toys.

Before you leave, stand still for one more moment. Feel the iron under the path, and the cool air rising from the river. Notice the colours of the narrowboats, the moss on the stone, the trees running down to the water. When you walk away, look back once. The line of arches stays in view for a long time... a thin thread of water across the valley, carried there by careful hands more than two hundred years ago.