An incredible £4.5 billion tunnel stretches underneath London and you probably had no idea it was there.

The Thames Tideway Tunnel is a deep-level 'super sewer' that spans 15 miles from Acton to Stratford, along the tidal section of the River Thames.

Work on the tunnel began in 2016, and tunnelling was completed in 2022. Construction was completed this year with the tunnel 7.2 metres wide and 67 metres deep.

It was part of a three-phase London Tideway improvements scheme to improve the health of the river. Sewage can be stored in the tunnel until it can be treated at Beckton.

The tunnel is designed to capture raw sewage and rainwater from sewers which would otherwise overflow into the river during heavy rain.

London's Victorian sewer network was designed to serve four million people. The city is now home to more than double that and is expected to double again in the next 100 years.

With even the slightest rain, the 19th-century network caused untreated sewage to spill into the Thames, adding up to tens of millions of tonnes a year and polluting the water.

Thames Water says the new tunnel will help provide habitats for a 'range of aquatic wildlife in the area' and slash the amount of sewage-related litter in the Thames.

"In 2016, we built the Lee Tunnel. It intercepts the tidal Thames' largest combined sewer overflow (CSO) at Abbey Mills," Thames Water said.

"A new company dedicated to building the Thames Tideway Tunnel was [then] set up. Bazalgette Tunnel Ltd, known as Tideway, started construction in 2016.

"In May 2024, the new 25km super sewer was connected to the Lee Tunnel. This completed the full 'London Tideway Tunnel' network.

"The new tunnel operates with the sewage treatment works and the Lee Tunnel. Together this new combined system has a combined capacity of 1.6m3 to protect the River Thames in London."

Since the Lee Tunnel's completion, Thames Water claims water quality in the areas has been found to improve.

A survey of the Channelsea River also found 14 different species of fish, which implies that the river is healthy enough to support a high-quality ecosystem and a wide range of fish.

The water company states on its website that following the tunnel's delivery, its wastewater customers will be contributing towards the cost through their water bills. It insists that annual household bills will rise by 'no more than £25 a year' (before inflation).

"An additional £2.5 billion has been invested by Thames Water for all three phases of this scheme," it added.