Three years ago, I was asked by a national newspaper if I thought it was possible to build a bridge or tunnel that could link Scotland with Ireland. I was intrigued by the question and did much research and later confirmed that it could be done. I told them how and from where. My response focused only on the architecture and engineering possibility, not the economics.

In early 2018, the Conservative government were still promoting the idea of a “Northern Powerhouse”, but “north” to George Osborne and Philip Hammond meant Manchester, Sheffield and Leeds not Scotland; the true north.

“It would be an investment in the true north.”

So, in my response, I added that such a link had the potential to create a "Celtic Powerhouse" and an investment in the true north. The next day it made the full front page and set in motion a proposal that has, since then, attracted professional and media headlines worldwide.

Initially, the idea of bridge or tunnel link united nationalist and unionist politicians on both sides of the Irish Border and on both sides of the North Irish Sea. Irish deputy prime minister Simon Coveney, DUP leader Arlene Foster, Scottish Brexit secretary Mike Russell and the then former Foreign Secretary Boris Johnson all gave their backing to the plan.

There were of course geological and infrastructure challenges but they were not insurmountable and there are international precedents that could be drawn upon. A road and rail crossing from Larne to Portpatrick would structurally and architecturally be possible and could boost tourism and trade for both sides of the crossing, while providing an extra needed physical connection, particularly after Brexit.

One of the major challenges was Beaufort’s Dyke: a deep-sea trench around 300m deep which runs parallel to the Dumfries and Galloway coastline that was used as a dumping ground for explosives after the end of the Second World War. However, a possible solution had been pioneered in Norway, using the concept of “floating bridges”, to overcome the depth and avoid column contact with the sea bed.

Norway has a population similar to Scotland and is in the process of investing £30bn to create the “Norwegian Coastal Highway”: a 1100 km route from Kristiansand in the south to Trondheim in the north. The road will cross 20 fjords, some more than 600 metres deep, using floating bridges and tunnel connections. The Norwegian Coastal Highway is a pioneering and remarkable infrastructure project and a sign of confidence for a forward- looking innovative country. Scotland and Ireland surely could achieve the same.

Another possible route for the bridge was between the Mull of Kintyre and Torr Head on the Antrim Coast, which is just over 12 miles apart. Here, a comparison with the Øresund Bridge was be made. The Øresund Bridge connects Copenhagen in Denmark with Malmo in Sweden.

Copenhagen has a population that is comparable to Glasgow and Malmo with Edinburgh. The Øresund Bridge was the result of a collaboration by both countries, each with a distinct, proud history but who “share a Nordic Cultural Heritage”.

“A road and rail crossing from Larne to Portpatrick would structurally and architecturally be possible and could boost tourism and trade for both sides of the crossing”

More than 25 million people use the crossing each year and the bridge has made a £7 bn return on the initial investment since its opening 18 years ago. It has also helped establish the Øresund Economic Region which employs 4m million people.

The case for the Celtic Crossing was not only about a return on investment, although it could bring financial benefit to the Ayrshire and Antrim coasts or Cowal peninsula. It was also about establishing closer social, cultural and political relations between Scotland and Ireland in the shifting political climate.

Clearly, politics in Scotland, Ireland, and in the rest of the UK is in an extreme state of flux; with questions over COVID, Brexit, border controls, and even independence still to be answered.

It now appears than a road and rail tunnel and not a bridge is favoured by Westminster and there are questions over differences in rail gauges between Scotland and Ireland, which in the scheme of things is the least problematic of the challenge that have to be overcome. There are many cross border countries that have different train gauges, Spain and France for example where variable gauge adjustable wheel sets are adopted. Same would be achievable between Ireland, Scotland and UK

I proposed that a road and rail tunnel could break through and emerge at Port Mora and Laird's Bay, two miles north of Portpatrick. That area of the Galloway coastline is defined by high sand stone sea cliffs which run into the sea to form a ledge below the wave live, before dropping where the sea gets much deeper. I think at this point a terminal can be constructed. My sketch drawings indicate how that would look. It is an architectural set piece, not an engineering proposal but the terminal is intended to suggest that it rises from the ledge and is sensitive to the landscape, context and sea front setting.

It is possible that the tunnel could loop around to the north of Beaufort's Dyke beyond the known munitions dump and continue down from that point to a level of 12 metres below the wave line, so ships can pass over. Pontoons, harnessed to the sea bed by cable ties, are set at 200 metre centres and support the tunnel below, with sea lanes openings of a greater width of 400 metres set at intervals. The pontoons would be designed to capture wave energy.

“Scotland and Ireland has the engineering and architectural talent and the capability to build this project.”

Oil rigs in the Gulf of Mexico are really like massive pontoons and they are connected to the sea bed, sometimes 1,000/1,500 metres below by steel cable ties and engineered to withstand hurricanes. I agree there are rough waters in the North Irish Sea but as far as I am aware, no hurricanes.

Scotland and Ireland has the engineering and architectural talent and the capability to build this project. It would be a transformative economic generator and a world first.

Governments across the British Isles now need to work together on a feasibility study into a bridge or tunnel connecting Scotland and Ireland. Research should be carried out to establish the economic and social benefits of the bridge and assess any geological and engineering challenges it would pose.

Professor Alan Dunlop February 2021