When it rains in London, our old Victorian sewers fill up.
If the excess water isn't discharged somewhere, the sewage could back up into homes and streets. So, instead, it's released into the River Thames through "combined sewer overflows".
Thames Water says that, in a typical year, 39 million tonnes of untreated sewage flushes into the river. And - as the population of London increases - this problem is set to get worse.
So, they want to build a massive new sewer to increase the capacity of the system - and help clean up the River Thames.
Watch Thames Water's video below for more information.