Inside the potash mine one mile under the North Sea
Boulby Mine, a potash mine in the North York Moors National Park, was built in the 1970s on the site of a former ironstone mine.
Cleveland Potash runs the site and produces nearly a million tonnes of potash a year, which is used mainly in fertilisers.
Peter Day went down Europe's second deepest mine - one mile below the North Sea, where there are 600 miles (1,000 km) of roadways leading to the potash face.
Hear more on In Business on BBC Radio 4 on Thursday 25th April at 20:30 BST.