A pool at a disused Derbyshire quarry, known locally as the Blue Lagoon, has been dyed black in a bid to deter people from swimming in it.
Despite the water at the quarry at Harpur Hill, near Buxton, having a pH level of 11.3 dozens of people still swim there.
Bleach has a pH level of 12.3 and signs warn swimmers the water could cause skin complaints.
Swimmers are also warned the water contains rubbish and dead animals.
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