A huge £7.4billion underground tunnel which will become Britain’s longest is currently being built, with expectations to launch in 2030. 

The title of the UK’s longest underground tunnel is currently held by the London Underground’s Northern Linewhich measures 17 miles. However the new Woodsmith Mine Tunnel will measure 23 miles when it’s completed, meaning it will take the title. 

Although it was initially projected to cost £1.1billion, the projected costs have now soared to £7.4billion. Unlike the London Underground, this tunnel won’t be used to transport members of the public. Instead, the aim of the project is to provide a new route to transport fertiliser underground. 

In fact the tunnel, managed by AngloAmerican, will also represent the largest private sector infrastructure scheme. Its objective is to extract polyhalite, a nutrient-dense fertiliser, from the North York Moors near Whitby to a processing facility in Teesside. 

Sirius Minerals previously owned the scheme and calculated the initial cost. However, reports from 2023 indicated the scheme had already exceeded budget. Tom McCulley, head of Anglo American’s crop nutrients division, previously said estimates of $9billion costs were “not too far off”, according to the Times. The route will run from an underground deposit near Whitby to a processing site in Wilton, near Teesside.

At the end of 2025, the scheme achieved a significant milestone in exceeding 30km in length, which is just over 18 miles. The tunnel boring machine (TBM), named Stella Rose, surpassed the longest continuous drive by a single TBM.

Woodsmith project director Andrew Johnson stated recently: “We are incredibly proud of this milestone. The Woodsmith Project is one of the most innovative mining developments in the world today and construction is progressing well.

“We currently employ 1,100 people in the area of which 75% are local – something we are incredibly proud of. We are also proud to have a small international workforce with the specialist expertise we need for this unique world-class engineering project that will provide employment for hundreds of local people for many many years to come.”

The tunnel proposal initially faced some opposition. Sirius Minerals successfully navigated 98 environmental requirements to secure planning approval for Britain’s first deep mine in over four decades.

The Northern Line, which currently takes the crown as the UK’s largest tunnel, runs the London Underground service between the north and south of the city. But what many people don’t realise is that it used to be two separate tunnels. 

City & South London Railway (C&SLR) ran the first deep-level tube railway with electric trains on what is now the Bank branch as far back as 1890, while the Charing Cross, Euston & Hampstead Railway (CCE&HR) ran a line that’s now the Charing Cross branch, which opened in 1907.

Initial plans to connect the two lines at Camden Town were scuppered due to WWI, but resumed in 1922, opening two years later as the Morden–Edgware line and becoming the longest tunnel in the UK. In 1937, it was relaunched as the Northern line, and the tunnel could have been even longer, were it not for the outbreak of WW2 which once again ruined expansion plans. 

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