Pontcysyllte Aqueduct

Thomas Telford did something no engineer had dared before: instead of a stone trough lined with clay — the only way anyone had carried a canal across a valley until then — he cast the whole watertight channel out of iron plates, 3.7 metres wide and 1.7 metres deep, and hung it 38 metres above the River Dee on eighteen hollow stone piers that taper toward the top to shed weight. He'd tested the idea on a smaller scale at Longdon-on-Tern first, then bet the Ellesmere Canal Company's money on it here. Sealing the iron joints was its own invention: the joints between the iron plates were packed with Welsh flannel soaked in a white-lead paste, then bolted tight — homely materials for a structure of such daring.

The trough has no towpath railing on the canal side — boats simply float past a sheer drop, with only a narrow raised path on the other edge for the horse. Two centuries of boaters have found this exhilarating or terrifying depending on temperament; it remains one of the most disorienting stretches of inland waterway anywhere, precisely because the water itself is the only thing between the gunwale and thin air.

At roughly £47,000, the project swallowed close to a tenth of the UK's entire annual output at the time — a scale of public spending that's hard to imagine committing to a canal bridge today. It worked anyway: the trough Telford poured in 1805 is still the one boats float through now, still watertight, still the highest and longest navigable aqueduct in Britain, which is why UNESCO listed it in 2009 not as a ruin but as working infrastructure.

Getting there

5 km from Rhosllannerchrugog. Liverpool John Lennon Airport (LPL) is about 44 km away.