Water at a disused quarry in Derbyshire has been dyed black to deter people from swimming there.
The action was taken at Middlepeak Quarry, in Wirksworth, to prevent accidents and deaths at the site.
The Mineral Products Association (MPA) said there were 15 deaths in quarry lakes in the space of two years in the UK.
The MPA, which is leading a national campaign about the dangers, said dying water was "very effective".
It said that immersion in cold water could dramatically affect even a strong swimmer's abilities.