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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Rain and low cloud will clear to the south leaving a dry night with light winds.
Winds to ease through the evening allowing temperatures to fall steadily with a widespread frost developing.
79-year-old Janet Jasper died after an explosion at her neighbour's house in Leicestershire.