A man has been killed after a "sudden gas blowout" at a potash mine in Boulby, near Cleveland in Yorkshire.
Cleveland police said officers had been called at 5.20am to an incident at the mine, during which a man died.
"Our thoughts are with family, friends and colleagues," police added.
"We are working to understand the circumstances surrounding the incident."
Boulby Mine has been operating in its present form since 1969, and was bought by ICL Group in 2002. The first potash was extracted from the mine in 1973.
The mine is also responsible for supplying around half of the de-icing rock salt, which is created as a by-product of potash, used on the UK's road system during the winter, according to ICL.
At 1,400 metres, it's the deepest mine in the UK.
ICL could not immediately be reached for comment.