Will Boris Johnson's 'bridge over troubled waters' between Northern Ireland and Scotland ever be built?

They are talking about the bridge again. Yes, that bridge. The imaginary one between Scotland and Northern Ireland.

Northern Ireland Secretary Brandon Lewis MP told Sky's Kay Burley@Breakfast it would benefit connectivity and the economy.

"Big infrastructure projects throughout history have sometimes been controversial, difficult, but they're the right thing to do," he said.

Some call the project backed by Prime Minister Boris Johnson "the unicorn bridge". Others refer to it as "the bridge over troubled water" and for good reason.

It is troubled by the Brexit reality of a border in the Irish Sea - between Northern Ireland and the rest of the UK - despite repeated government denials.

A command paper on the implementation of the Northern Ireland protocol has confirmed that there will be checks on some goods.

Sammy Wilson MP, the DUP's Brexit spokesman, claimed the Port of Larne in County Antrim had been instructed to prepare for Brexit-related checks.

Critics suggest the government is continually building this fantasy bridge to shift the focus from the customs post that is being built.

When Theresa May lost her Commons majority in 2017, she sent her people to ask the Democratic Unionists for their support.

Legend has it that one prominent DUP MP joked that the price of their votes would be a bridge between Scotland and Northern Ireland.

A DUP source, who had been in the room, told me "the blood drained from their faces because they thought we were serious".

Since then, the project has been resurrected every time the government needs to steady Unionist nerves in this corner of the UK.

There are many reasons why every mention of "the bridge" is met with a collective eye-roll, even by those who long for closer ties with the UK.

The most direct route - 28 miles - includes Beaufort Dyke, a trench containing more than a million tonnes of unexploded munitions, plus nuclear waste.

James Duncan, a retired offshore engineer from Edinburgh, said it was comparable to "building a bridge to the moon".

He said: "It would require about 30 support towers at least 1,400 ft high to carry the road deck across the deepest part and above the shipping channel.

If the Forth Road Bridge in Edinburgh and Foyle Bridge in Londonderry are any guide, high winds would close this sea bridge on an almost daily basis.

Then there is the cost - estimated at a cool £15bn. That's 15 times the amount the Tories paid for the DUP's support back in 2017.

It would be fair to say Northern Ireland's First Minister Arlene Foster has expressed more enthusiasm for the project than her Scottish counterpart Nicola Sturgeon.

It was 1869 when Irish engineer Luke Livingston Macassey first proposed a bridge between the two countries.

One hundred and fifty years later, there is still no bridge in the Irish Sea but there is every reason to believe there will soon be a customs post there.