Worried Doncaster quarry bosses are stepping up security and adding dye to the water in a bid try to stop a wave of summertime swimmers.
Bosses at FCC Environment, which looks after Skelbrooke Quarry and its lake, are concerned about the danger to swimmers there.
Now black dye will be put into the water to try to deter people from swimming.
Neil Swift, site manager, said: “Following several incidents of trespass at the disused Skelbrooke Quarry we have introduced around-the-clock security patrols and have dyed the water to make it look less appealing to illegal swimmers.
“We are also writing to local schools encouraging them to warn their children about the hazards of swimming in unsupervised waters like quarries, rivers, lakes and reservoirs, and of the dangers of ‘tombstoning’.
“We would like to emphasise swimming at unsupervised sites like this is dangerous, as the water is still exceptionally cold and hidden rocks lie just beneath the surface.
The action comes almost a year after a swimmer drowned taking a dip in another of the borough’s quarries.
Mr Swift said: “Sadly, there are a number of tragic drowning accidents in the UK every summer, and there has already been a death earlier this year at a quarry near Wigan.
“It is also important people are aware trespassing onto private land is against the law.
“We will continue to work with the police on this issue, and anyone seen intruding on our site will be photographed and any vehicle details recorded with a view to prosecution.”
The Canal and River Trust put out a warning last week warning youngsters swimming in a Doncaster canal they were dicing with death by doing so.
A spokesman said today: “We know the hot weather might make it tempting for people to cool off in the water. Our message is it is not always safe to do so.
“We’ve had reports of people swimming at the Don Doors aqueduct on the New Junction Canal, which goes over the River Don in Kirk Bramwith.
“We have sent a team out to the site to see if there is anything further we can do to discourage people from swimming there.
“We’d always advise people to not swim, because it might not be safe.”
The warnings comes almost a year after Matthew Mellor drowned swimming in a quarry in Auckley.
The 27-year-old, from Doncaster drowned in the old quarry off Hurst Lane, Auckley, after going swimming with friends on July 21.
He was having an early evening swim with eight friends.
And a warning of the dangers of swimming in open water went out when 15-year-old Philip Law died at Manvers Lake, Wath, in 2010, after getting into trouble while swimming with friends there.