Updated:We may watch the fat in our diet. But give little thought to what happens to waste cooking oil and fat - especially when it's poured down the drain. Deep below London's streets, the results are all too clear. This is what happens to leftover fat which goes down the plughole. It cools and becomes fatbergs, which clog up the city sewers. UPSOUND "Disgusting, yeah it is" Now the waste oil and fat is to be used to power a new electricity plant, making life easier for flushers like Rob Smith.