It's known as the 'Blue Lagoon' and it's tranquil turquoise waters draw people from far and wide.
But the flooded former quarry near Buxton hides a toxic secret. It's so polluted that it's almost as toxic as bleach.
There are warning signs that state the water can cause skin and eye irritations and stomach problems some people are ignoring the signs and swimming in it.
The ph level of pure water is 7.
The ph level of this household bleach is just over 11.
And the ph level in Blue Lagoon is 10.8.
The council says they're now looking to drain it.