About midway across the proposed route lies a deep underwater trench called Beaufort's Dyke. It is roughly 30 miles long and reaches depths of more than 300 metres. After the Second World War, the British military used it as a dump for surplus munitions - by some estimates more than a million tonnes of conventional ordnance, plus chemical weapons and, at one point, decommissioned nuclear waste containers. For decades the assumption was that the stuff stayed at the bottom. It does not, entirely. Phosphorus incendiary devices have washed up on Scottish and Irish beaches over the years, sometimes injuring people who mistook them for stones. Any bridge or tunnel crossing this water would either have to bridge above the dyke, accept that piles cannot go through it, or undertake one of the largest underwater munitions clearances ever attempted. Architect Alan Dunlop suggested floating sections of bridge on deep sea orbs anchored by tension cables, similar to engineering used on Norwegian fjord crossings. None of it was simple. None of it was cheap.

Politicians across both islands, perhaps surprisingly, lined up to back the study. Scotland's First Minister Nicola Sturgeon supported it. So did the Irish foreign minister Simon Coveney. So did Northern Ireland's Arlene Foster and Taoiseach Leo Varadkar, who pointed out that people had once dismissed the Channel Tunnel and the Oresund Bridge as fantasies and they got built. Boris Johnson - then Foreign Secretary, then Prime Minister - was the project's most enthusiastic champion. He commissioned a cost study from civil servants in 2019. He recommitted to it through 2020. The first concept estimates put the cost at £20 billion. Then £15 billion. Then engineers got hold of the numbers and the totals climbed rapidly. By February 2020 government officials had begun scoping out a full feasibility study. Dominic Cummings, Johnson's former chief advisor, later told the BBC that the bridge was the prime minister's only real agenda - 'build the world's most stupid tunnel to Ireland.'

The full feasibility study, conducted by two senior engineers at the recommendation of Sir Peter Hendy's infrastructure review - Douglas Oakervee, former chairman of HS2 and Crossrail, and Gordon Masterton, formerly of Jacobs Engineering - was published in November 2021. The cost of the review itself was £896,681. Their conclusion was that the technology existed: a bridge would be the longest-span bridge ever built; a tunnel would be the longest undersea tunnel ever built. Either could be done. A bridge would cost £335 billion. A tunnel would cost £209 billion. Construction would take 30 years and require very significant road and rail upgrades on both sides. The engineers concluded that the cost in either form was 'impossible to justify' and recommended that further work on the fixed link should not progress beyond their study. By then, sources in the Treasury were already telling the Financial Times the project was 'dead, at least for now.'

The report did note what the bridge would have created. More than 35,000 jobs in design and construction. A potential carbon-neutral structure within 40 to 60 years of completion, with the bridge platform doubling as a wind, tidal and solar power generator. A tourism corridor between Northern Ireland and Scotland. The replacement of seabed utility cables and pipelines with services routed through the structure itself. The London-to-Dublin air route, the busiest in Europe, would have lost passengers to rail. The A75 across southern Scotland would have had to be upgraded. The two rail networks - Ireland uses 1,600 mm gauge, Britain uses 1,435 mm - would have needed dual-gauge rolling stock or break-of-gauge transfers. But none of that happened. The proposal sits on the same shelf as the Channel Tunnel proposals of the 1880s, the second Channel Tunnel of the 1920s, the various Bristol Channel barrages of the 1950s - British infrastructure plans that get studied and rejected and then revived again decades later by the next government that wants something visible to point at. The North Channel still has the same depth. The same dumped munitions. The same ferry crossing. And the same 45 kilometres of open water that no one, yet, has decided is worth the price.