The fields around Stalmine and Preesall, south of Knott End, exhibit an unusual land use: salt extraction for the chemicals industry. The red objects just visible in the middle of the field on the left are brine wells, which look disused.
Salt deposits, formed by the evaporation of a Jurassic inland sea, were discovered in the 1870s. Extraction was (and is?) generally achieved by pumping fresh water into the deposits, allowing the salt to dissolve, then pumping the resulting brine to ICI Thornton for the production of chlorine. Mining (with explosives, 100-300m underground) also provided ~12,000 tonnes of high-quality rock salt per month at the end of the 19th Century, which was exported worldwide. Local subsidence became a minor tourist attraction!
A more recent issue is the proposal to rapidly dissolve large caverns into the salt deposits (simply discharging the saline solution back to the sea) and use the resulting voids for underground storage of natural gas. Local residents are concerned. I don't know enough about the issue to comment on whether they have reason to worry or it's just nimbyism.
The warning sign is oddly permissive: unauthorised personnel aren't obliged to enter, but they may if they wish?
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