Beaufort's Dyke is a natural feature, despite its straight-edged name. It is a submerged tunnel valley, carved into the seabed by glacial meltwater during the last ice age and kept clear of sediment ever since by strong tidal currents through the North Channel. The trench runs fifty kilometres long, three and a half kilometres wide, and reaches depths of around 300 metres. From a chart it looks like a knife slash drawn through the floor of the channel between Belfast Lough and the Galloway coast. Captain Frederick William Beechey of the Royal Navy is credited with its discovery in the early nineteenth century, although the trench was already recorded in 1856 as having been found some years earlier. For seventy years after that, it was simply a deep place on a chart. Then it became a place to throw things away.

The military port of Cairnryan, opened on the Scottish side of the channel during the Second World War, made the Dyke uniquely useful. After 1945, Britain found itself with vast surplus stocks of weapons and chemical agents and very few good options for disposing of them. The Ministry of Defence has acknowledged that well over a million tons of munitions were dumped, including the July 1945 disposal of 14,500 tons of five-inch artillery rockets filled with phosgene, a choking agent that had been used to terrible effect in the First World War. The dumping continued into the 1970s. The decisions were made under wartime secrecy and post-war pragmatism. The consequences were left for later generations. Later generations have started to arrive.

In 1995, phosphorus bombs began washing up on Scottish beaches. The timing coincided with the laying of the Scotland-Northern Ireland Pipeline, a 24-inch gas interconnector built by British Gas. The pipeline crossed the southern edge of the dump area and apparently disturbed material that had been resting on the seabed for half a century. Over the preceding five years, anti-tank grenades had appeared on shores in Northern Ireland and the Isle of Man. The munitions are still there, slowly corroding. Phosgene, in particular, was packed into shells with rubber seals that have been degrading for eighty years. Bomb-disposal teams in Scotland and Northern Ireland have a quiet, steady stream of work that the rest of the country mostly never hears about: a fisherman finds something strange in his nets, a child picks up a shiny metal cylinder on a beach, and a careful phone call is made.

In February 2020, the UK government announced a feasibility study for a fixed crossing between Northern Ireland and Scotland. Two routes were considered: Portpatrick to Larne, and Kintyre to Torr Head. The Portpatrick option would cross Beaufort's Dyke. The engineers concluded that even at the narrowest point, a bridge would need spans approaching four kilometres on foundations set well back from the trench's edge, because no one really wanted to start drilling anchor piles into a million tons of unexploded ordnance. The estimated cost reached 335 billion pounds. The project was rejected. The Dyke had outlasted yet another scheme to bridge it. Boris Johnson's government had spent around a million pounds on consultancy fees by the time the proposal was quietly shelved. The trench remained as it has been since the ice melted: a deep cold place where nobody really wants to look too closely.