Several teenagers have become ill after they tried to beat the heatwave by cooling off in a makeshift pool filled with river water.
Dozen of young people went for a dip after a skate park in Hebden Bridge, West Yorkshire, had somehow been filled with water from the nearby River Calder.
Pictures show at least 20 young people swimming in the dirty, brown water during the recent heatwave.
But the decision backfired when several have since fallen ill and are believed to be suffering from sickness and diarrhoea.
The skate park has since been drained and Calderdale Council has launched an investigation to determine exactly how the river water ended up there. It is understood the water may have somehow been pumped into the concrete crater rather than it being filled by hand.
A nearby high school, Calder High in Mytholmroyd, has written to parents saying a number of students had become ill after taking part in what was 'not an officially organised activity'.
Environmental Health officers are advising anyone with concerns to contact their GP. Emergency services across the country have warned against swimming in open water after a spate of deaths in the hot weather.
Daniel Clemo, 24, drowned while cooling off in a reservoir in the Brecon Beacons, South Wales, on Sunday. And a 16-year-old boy and a 41-year-old man both died in separate incidents while swimming in a quarry near King's Lynn, Norfolk.