As London Mayor, Boris Johnson developed a knack for promising — and sometimes delivering — infrastructure that critics deemed inefficient and expensive.

Key points: Plans to link Britain and Ireland by bridge have been mooted since the 1800s

Plans to link Britain and Ireland by bridge have been mooted since the 1800s Boris Johnson has asked the civil service to assess a bridge's costs and risks

Boris Johnson has asked the civil service to assess a bridge's costs and risks Experts say that the bridge has the potential to heal British tensions over Brexit

Critics have called them Mr Johnson's "vanity projects": such as the failed Garden Bridge over the Thames, the rollout of hybrid-diesel London double-decker buses that initially malfunctioned (since discontinued), and the world's most expensive cable car running for a kilometre in East London.

Today, it appears that history may be repeating at a national level — this time with a bridge linking Northern Ireland and Scotland.

In September, Britain's Channel 4 revealed documents which showed that both Britain's treasury and transport departments were asked to advise on the costs and risks of a possible bridge.

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There are sizeable obstacles, considering the sea between the two countries is a literal minefield and dumping ground for radioactive waste.

But what would a fixed-land connection across the British Isles look like? And what benefits could it bring to a Brexit-riddled Britain?

Beyond politics — getting the bridge past Boris

The timing of Mr Johnson's support of a bridge may be influenced by his desire to sell the idea of an "energised" post-Brexit Britain ahead of a national election later this year.

Architect Alan Dunlop first proposed the idea of a 'Celtic Crossing' between Scotland and Northern Ireland for Scottish newspaper The National in 2018 in the hopes of boosting economic ties in the north.

The Larne–Portpatrick route is preferred because of its connection to existing infrastructure. ( ABC News: Jarrod Fankhauser )

The proposal cited the route most pundits prefer — a 32-kilometre link between Larne on Northern Ireland's east coast and Portpatrick on Scotland's west coast.

Larne is about 30 minutes from the Northern Irish capital Belfast, while Portpatrick is about two hours from the closest Scottish and English cities, Glasgow and Carlisle.

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Professor Dunlop told the ABC his proposal wanted to "stay well clear" of "rabid" British politics and look at the idea from a purely practical perspective.

"Unfortunately, a lot of the negative comments about [the bridge] seems to be coming from people who are absolutely against Boris Johnson and the idea of Brexit," Professor Dunlop said.

For other proponents of the bridge, such as Alan Pert, the Scottish-born director of the Melbourne School of Design (MSD), Mr Johnson's involvement shouldn't stop the bridge idea from being looked into.

"Boris doesn't have a good track record with bridges, never mind infrastructure — as soon as he puts his name to something you get a political backlash," Professor Pert said.

"To announce a bridge connection, to talk up the political and symbolic importance of such a link at this point in time — you really can't underestimate the impact of it."

The sea below is full of bombs and radioactive waste

At a length of 32 kilometres, the project would sit in the top 10 world's longest sea bridges, but where the bridge may be breaking records lies beneath the water — both in terms of the sea depth as well as the fact that it holds a million tonnes of weapons and waste.

The seafloor between Northern Ireland and Scotland is incredibly deep, because of a 3.5-kilometre-wide trench known as the Beaufort Dyke.

A bridge connecting Northern Ireland and Scotland would span one of Europe's most dangerous sea trenches. ( ABC News: Jarrod Fankhauser )

Scientists have speculated it was created from a series of glacial melting events, where it plunges 312 metres at its maximum depth — making it one of the deepest areas in the European continental shelf.

Any kind of structural supporting tower in that part of the dyke would almost be the height of the Eiffel Tower, which is 324 metres from base to tip.

However, Professor Dunlop told the ABC that an ideal bridge route would be to the north of the dyke's deepest point, where the seafloor is at 150 metres below sea level.

The seaside town of Portpatrick on Scotland's west coast, is touted as the bridge's British link. ( Flickr: Mundus Gregorius )

The British Ministry of Defence estimated that a million tonnes of unexploded munitions were dumped in the Beaufort Dyke between 1920 and 1972, following repeated inquiries from Scottish MPs in the mid-1990s.

This came after more than 4,500 of incendiary bombs — which may ignite after drying out — started washing up on Scotland's coast in 1995.

In 1997, it was revealed that almost two tonnes of radioactive waste was also dumped into the dyke.

"What on earth are we doing as a country with a sea trench filled with Second World War munitions so close to both our coastlines?" Professor Dunlop said.

He said opponents used the dumped munitions as a reason to cancel the bridge idea entirely, but instead, he hoped the bridge debate could trigger a broader national conversation about what to do with the waste and munitions in the dyke.

"This idea that we have this sea trench filled with munitions demands serious debate," he said.

While a technically easier place to build a bridge lies to the currently suggested route's north, those plans were sidelined due to its landing points being too remote to make travel feasible.

So how much might this all cost and in what form?

The Danish-Swedish Oresund Bridge has been touted as one case study for the British bridge. ( Flickr: Håkan Dahlström )

Nobody knows what form the final bridge may ultimately take.

Professor Dunlop's draft sketch of a bridge for The National shows a cable-stayed bridge — a bridge with one or more towers which carries cables tethering it to the road — stretching across a body of water.

He told the ABC the engineering would "not be a walk in the park", but added his preliminary ideas were done in the hope to trigger a "serious" feasibility study.

Abroad, he cited built examples such as the Oresund Bridge — linking Copenhagen in Denmark and Malmö in Sweden — and the world's longest sea bridge linking Hong Kong with Macau as visions of what an Irish Sea bridge might look like.

He said Norway's coastal highway was another example, which involves connecting the country's rugged west coast with numerous bridges and tunnels, including a "floating tunnel" — where dual road carriageways would be submerged and tethered to floating pontoons.

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Professor Dunlop estimated that the bridge itself could cost between 15–20 billion pounds ($28–37 billion) — a number that he acknowledged didn't take into account the budget needed to upgrade the road networks to and from the bridge.

"I'm an architect, not an economist," he said.

Esmond Birnie, a senior economist at Ulster University, estimated that road upgrades on either side of the bridge could add another billion to the overall price tag.

By comparison, the overall cost for the 11-kilometre Oresund bridge — including its peripheral infrastructure — cost $6.5 billion, while China's Hong Kong-Macau bridge cost $20 billion.

Dr Birnie wrote that any pan-British bridge would make an "unlikely commercial proposition".

Melbourne-based design director Pert said that any kind of economic feasibility study would have to be done to assess the opportunities the project may have for de-industrialised towns on either side of the bridge that "have been disenfranchised for the past 40 years".

He added that it was the role of architecture to visualise things in order to mitigate against a community's "fear of the unknown".

"By illustrating [the bridge] — rather than showing it as a line on a plan — it's managed to change the conversation ," he said.

What would a bridge mean for post-Brexit Britain?

The Anglo-French Concorde has been cited as a example of a project worth breaking the budget for. ( Reuters )

If the bridge idea does get the green light however, it would occur in a post-Brexit world where the British economy may be poorer.

Even if the UK manages to leave the European Union with a free-trade-style deal similar to the one that has passed the House of Commons, the UK could still see a 6.7 per cent drop in GDP by 2034, according to data from the country's Office for Budget Responsibility.

And while 20 billion pounds ($37 billion) might be small change for the UK — its GDP was $4.09 trillion in 2018 — the idea of splashing cash on a project that may not be commercially viable might be a bridge too far for a country that is still reeling from almost a decade of budget austerity.

Dr Birnie also said budgetary concerns "need not imply that a bridge should not be built", but instead planners should consider "broader catalytic or transformational effects and how big they might be".

"The Concorde supersonic aircraft and post-War British atomic power stations may have been engineering marvels but they were far from commercially viable and might well have 'failed' the test of an economic cost-benefit analysis," he wrote.

Professor Dunlop cited the example of southern France's award-winning Millau Viaduct, which is the tallest bridge in the world standing at just over 336 metres, as an example of transformative infrastructure.

"The piers for that are 20 metres higher than the Eiffel Tower — so as a piece of world-respected engineering it's really put that area on the map," he said.

The piers for the France's Millau Viaduct are taller than the Eiffel Tower. ( Flickr: Steve Harris )

When asked about what possibilities a bridge may bring to cross-Irish sea relations, a spokesperson for the Scottish Government told the ABC they were "always keen to talk about how we can strengthen connections between Scotland, Northern Ireland and the Republic".

"There are an obvious number of practical obstacles and challenges to such a concept, and it would require a robust assessment of the costs or benefits of such a project in the first instance," they said, adding that Edinburgh currently had no plans to consider bridge proposals.

In Dublin, Irish Prime Minister Leo Varadkar told reporters the proposal shouldn't be "dismissed out of hand" and called for it to "be at least examined" earlier this week.

In Northern Ireland, the bridge has support from the British loyalist Democratic Unionist Party (DUP) but has been criticised by the territory's largest Republican party, Sinn Fein.

Beyond a revitalisation of the region's fortunes, Professor Pert said the bridge could serve a much greater purpose at a time where the unity of the United Kingdom appears to be fraying at the seams.

"In Scotland, the view is that England's tearing itself apart … but it's Britain that is tearing itself apart— it's quite frightening," Professor Pert said.

"I can't help but feel the symbolism of this would be huge for Northern Ireland and for Scotland … I'm convinced of its power."

The UK Prime Minister's office and the office of DUP leader Arlene Foster were contacted for comment.