BBC: Why has Windermere lake turned green?
"Windermere in Cumbria is England's biggest lake and attracts thousands of tourists every year.
But in recent months, people have been advised not to swim in it or allow their dogs to go in the water in it as it has seen a huge increase in pollution levels.
Conservationists say this is due to raw sewage (what humans flush down the toilet), being pumped into the lake as the nearby water treatment site is struggling to handle the amount of excess storm and rain water.
Storm overflows are designed to discharge sewage into rivers, or the sea, at times of heavy rainfall or snow melt to prevent it backing up into homes and streets.
But it appears they are being used far more frequently than they should.
This raw sewage is also causing problems for the wildlife around the lake."
