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.
More top news
Paramedics were on standby outside the police cordon as armed officers stood by the property.
A team of researchers has taken samples from the ten busiest London Underground lines to work out which are the dirtiest.
The action comes on top of an emergency timetable which has seen hundreds of trains a day being axed.