Bomb disposal experts have cast doubt over UK Prime Minister Boris Johnson's ambitious plans to build a 28-mile bridge between Northern Ireland and Scotland, saying that it would be too dangerous due to a vast munitions dump on the bed of the Irish Sea. 

Any bridge that spans between the two countries would likely have to traverse that site, experts say. 

The most direct route between the two countries would involve spanning Beaufort's Dyke, a munitions dump containing more than one million tonnes of unexploded bombs, in addition to chemical and nuclear waste. 

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The site is about 31 miles long and as deep as 300 feet in places. 

British ships dumped guns and bombs at the site from the 1920s to the 1970s and it contains German and Allied weapons from the second world war. 

Robin Rickard, a consultant with the explosive ordnance advisers Exord, said that the work involved in constructing the bridge would "pose an unacceptable level of risk."

He told the Guardian: “In my opinion, this would be without doubt an enormous undertaking. Without knowing the planned design and engineering methodology, but assuming it would involve deep intrusive seabed works in proximity to unexploded ordnance, it would thereby represent a significant risk to the project and its personnel.”

The dump site is not monitored regularly by the British Government and the construction of a gas pipeline near the site two decades ago resulted in incendiary bombs from World War Two being washed ashore, with some of the bombs exploding when they dried out. 

Deidre Brock, a Scottish MP, said that she wanted the British Ministry of Defense to conduct an eco-audit of the site to assess its condition. 

“I asked how the MoD monitors this ticking timebomb but it turns out no one monitors it and nobody knows what condition it’s in. No one knows what’s down there and no one looks at it. How could anyone think it was sensible to build a bridge over all that?” she said.

The Ministry of Defense says that it has a moral responsibility for the site since it dumped the munitions there in the first place, but it maintains that the site is safe for the construction of any bridge, a stance backed by a survey conducted in the 1990s. 

The idea for a connection between Scotland and Northern Ireland has been raised on several occasions in the past and was suggested by the Center for Cross Border Studies, a thinktank, more than a decade ago.

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At the time, a bridge was estimated to cost roughly £3.5 billion. 

Now, however, conservative estimates place the cost of any bridge between the two countries at over £20 billion after Boris Johnson touted the idea last fall. 

It is thought that the bridge would connect the Northern Irish town of Larne with the Scottish coastal town of Portpatrick if it was ever built.