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.
Image courtesy of Alix Williams, CC BY-ND 2.0
