For decades London’s Victorian-era sewers have discharged raw sewage into the River Thames after heavy rain.
But that could be about to change, thanks to a vast tunnel that’s been built underneath the river.
Seven metres wide, 25km long and costing nearly £5bn it’s designed to divert sewage to a treatment plant in east London.
Our environment correspondent Jonah Fisher was given a rare opportunity to ride a bike along the tunnel before it fills up with sewage.
Producer: Sophie Woodcock
Filming and editing: Kevin Church