The Blue Lagoon near Buxton, a flooded quarry - has inviting turquoise waters, but it is so polluted it's almost as toxic as bleach.
There are warning signs saying it can cause skin and eye irritations and stomach problems, but families and young swimmers are ignoring them.
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West Midlands Weather: Outbreaks of rain overnight and continuing into Wednesday morning.
East Midlands Weather: Heavy rain overnight which will linger into Wednesday morning.
Sian Smith's son, Sam, and Katie Woodhall's son Toby, are good friends, and attend Coughton School in Warwickshire.