New pictures show engineers tunnelling deep under London to create a 25km (15.5 mile) super sewer.
The Thames Tideway Tunnel will run from Acton in the west to Abbey Mills in east London where it will join up with the Lee Tunnel which will take all the sewage to be treated.
The tunnel aims to prevent tens of millions of tonnes of raw sewage polluting the River Thames every year and is expected to be completed by 2024, Tideway said.
These new pictures were taken at the one of the main digging sites and show workers continuing to tunnel deep beneath the surface to construct the super sewer.
Work is under way at 23 sites across the capital with four tunnel boring machines (TBM) being used to burrow deep beneath the surface, Tideway said.
Tideway said that the tunnel will go from 30m deep in the west to 50 or 60m deep in the east and will mainly follow the course of the River Thames.
Work began on the super sewer in 2016 and the project is expected to cost £3.8 billion, Tideway said.
So far 6km of tunnel has been built, with four TBMs in the ground, while 1.6 million tonnes of rubble and debris have been removed by river, saving 100,000 lorry trips, it added.
Spokeswoman Hannah Shroot said that the tunnel is at a gradient so that the sewage does not need to be pumped through.
She added: “At the moment millions of tonnes of raw sewage pour into the Thames every year.
“The Victorian sewers are still in perfect condition but because the population of London has grown so much they are full to capacity and regularly overflow into the river.
“The tunnel we are building is the solution to that – it’s a 25km sewer tunnel from Acton to Abbey Mills in east London.
“There it connects into the Lee Tunnel which will take all the sewage to be treated.
“The tunnel goes from 30m deep in the west, and the deepest it gets is 50-60m in the east.
“It is at a gradient so it flows by gravity, there’s no pumps in the actual tunnel.
“It’s a lot deeper than the current sewers.”