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Video: Inside London's Deepest Tunnel

We're standing on London's deepest railway tracks, 75 metres beneath the surface. By the time you read this, the trains will be gone, replaced by a river of raw sewage that would easily swamp a double-decker bus. This is the Lee Tunnel, a brand new sewer running from Abbey Mills to Beckton.

Every year, 16 million tonnes of untreated sewage discharge straight into the River Lea. The tunnel is designed to capture that effluent before it enters the river. A gentle slope will then carry the feculent soup 4.3 miles east to Beckton for treatment.

The Lee Tunnel will eventually connect to the more famous Thames Tideway Tunnel, a new interceptor sewer, built on an even larger scale to greatly reduce sewage discharge into the Thames.

We took a tour of the Lee tunnel at the Beckton end, shortly before it was put into commission. Watch the video above, or see the captioned photos below to learn more about the project.