Inside the Yorkshire mine where England’s gritters get their salt

Over time, the seawater evaporated, leaving vast salty deposits that were gradually covered over.

Nowadays, some of those rock salt deposits can be found at a depth of about 330ft (100m), while others are much deeper – well over a mile (1.6km) underground.

Thomas Edwards, ICL Boulby’s chief geologist, said the salt was “hauled out by people working 24 hours a day, in the dark, beneath the sea”.

Rock salt is used for winter highway maintenance as salt lowers the freezing point of water to below 0C (32F).

This means that when salt is spread over a road or a footpath, it either melts the snow and ice as it dissolves, or it helps prevent ice forming.

Mr Edwards said during any cold snap, most gritters in the north of England would probably be filled by salt mined at Boulby.

“I like to tell people that when their cars are a little bit grubby when the roads have been gritted in the winter, that’s 260 million-year-old dirt that’s dirtying your cars,” he explained.

“It’s really quite something to think about. And a bit of a positive rather than, ‘what’s all this grub on my car?’.”