IT’S an idea that has fascinated engineers and politicians on both sides of the Irish Sea for more than a century with Prime Minister Boris Johnson now asking government officials to explore the possibility - and the finances - of building a bridge between Scotland and Northern Ireland.
Both the Treasury and Department of Transport have been asked for advice on the costs and risks of such a project.
It is not the first time that Mr Johnson, a lover of large- scale infrastructure projects - has shown an interest in the bridge, which would potentially run from either Portpatrick or the Mull of Kintyre on the Scottish side.
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The idea was first raised by him last year when he was Foreign Secretary.
"This PM has made no secret of his support for infrastructure projects that increase connectivity for people and particularly those that strengthen the Union," a Number 10 spokesman said this week.
Now, with Brexit firmly in sight, the bridge could help smooth out one of the toughest political challenges of the day.
READ MORE: Boris Johnson: Bridge between Scotland and Northern Ireland 'would only cost £15bn'
The Democratic Unionist Party has previously suggested that such a bridge would go a long way to breaking the Brexit deadlock, as it would negate the need for a border in the Irish Sea.
Last year, East Antrim MP and leading DUP member Sammy Wilson last year compared the importance of building a fixed crossing over the North Channel of the Irish Sea to the high-speed rail project underway in the south of England.
He called for a feasibility study on the subject, and added the idea of building the Channel Tunnel to France was once widely derided.
A 2007 report by the Centre for Cross Border Studies estimated building a bridge from Dumfries and Galloway to Ulster would cost £3.5 billion although since then estimates have risen to £10bn.
This week, Mr Johnson said it would "it would only cost about £15bn."
There are two potential routes regularly suggested for a crossing. A Portpatrick to Belfast Lough link would be around 21 miles in length, while Antrim to Mull of Kintyre just 12 miles.
The latter option is routinely discounted however, as the road network from Campbeltown on the Scottish side would require significant upgrading through mountainous terrain, and lacks a direct rail service.
But any bridge or tunnel between the two countries would face other logistical challenges.
One of the biggest obstacles is Beaufort’s Dyke, a 31-mile long sea trench more than 200m deep.
It lies six miles from the Scottish coast and was used as a dumping ground for conventional and chemical munitions in the aftermath of the Second World War.
Bridges covering longer distances have been built elsewhere, but engineering experts are sceptical that such a crossing could be built over the North Channel.
Ronnie Hunter of the Institution Of Civil Engineers said a rail tunnel would be a more likely option.
“The North Channel is something like 32,000 metres coast-to-coast, where as the Forth Road Bridge is around 2,500 metres in comparison,” he said.
“If you’re talking about building a bridge, it would be multi-span and require dozens of piers across the channel.
“There are numerous bridges in North America built across relatively shallow water which go on as causeways for mile after mile. But we’re not talking about shallow water here - this is essentially next to the Atlantic Ocean, in very deep water.”
The Scottish Government has pointed to the strong transport connections between the two countries and highlighted investment that both P&O and Stena have made improving the ships and ports on the Loch Ryan to Larn and Belfast in recent years.
The government has invested £58m in the M77 and A77, the main routes taken to the port, since 2007.
Last year, architect Professor Alan Dunlop called for the Scottish Government to work with counterparts in Ireland and Westminster to undertake a full feasibility study into the major infrastructure project.
He produced an early sketch of his vision how the bridge would look and pointed to the success of the Øresund Bridge that connects Denmark with Sweden. It has made a £10bn return on the initial investment since its opening 18 years ago.
Around 17,600 people, including 2,500 students, commute across the bridge everyday.
Prof Dunlop said: “We have the engineering and architectural talent and the capability to build this project; it would be a transformative economic generator and a world first.
“ Wanting to reduce the border between Scotland and Ireland seems to be highly relevant at the moment.
“The relationship between UK, Scotland and Ireland seems to be in a state of flux.
“This seems to be an opportune moment to talk about how a major infrastructure project could effectively break down borders.”