The pool, desribed by many as DANGEROUS was dyed black in a bid to put people off swimming in it.
The water at the quarry at Harpur Hill, Buxton, has a pH level of 11.3 - bleach has a pH level of 12.6 - but dozens of people had still been swimming there. A sign at the pool, known locally as the blue lagoon, warns people that its polluted water includes "car wrecks, dead animals, excrement and rubbish".
Another says the water's high pH levels can cause "skin and eye irritations, stomach problems, fungal infections". It adds: "Would you swim in ammonia (pH 11.5) or bleach (pH 12.6). Well these are similar to the Blue Lagoon (pH 11.3)."
The signs were put up by local authority High Peak Borough Council. The lagoon's natural blue colour came from caustic chemicals in the quarry stone. Some people are happy at the change while others not so, with one comment that "It is a grave shame to replace a blue lagoon of beauty to a black pit of disgustingness."
Below, an image of the Blue Lagoon when it was originally Blue and the now Black Lagoon.