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.
A High Peak Borough Council spokeswoman said the pool contained vehicle parts, "all sorts of rubbish" and warned that the water temperature was "extremely cold".
Signs put up by the council last year around the previously bright blue water - which gets it colour from the caustic chemicals in the quarry stone - warn people it could cause skin irritations and stomach problems.
The signs also state there are vehicles, rubbish and dead animals in the water.
Local business owner Rachel Thomas said she thought changing the colour of the quarry water had made a difference.
"It's not pretty any more," said Ms Thomas.
"They don't think they're on holiday in the Bahamas any more, they know they're in Harpur Hill.
"It was absolutely beautiful to look at but was horrendously dangerous."