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Beneath the River Thames runs a tunnel stretching about 25 kilometres. Most people will never see it, yet this tunnel now captures large volumes of sewage that would otherwise spill into the river. Known as the Tideway Tunnel, it runs beneath the river from Acton in west London to Abbey Mills in east London, before directing flows to the Beckton sewage treatment works. The project cost about £4.5 billion and has often been described as London’s “super sewer”.

To understand why this tunnel is needed, it helps to look at London’s original sewer system. Much of the city’s main sewer network was built in the nineteenth century during the Victorian era, designed by the engineer Joseph Bazalgette. At the time London was struggling with repeated cholera outbreaks and severe river pollution. Bazalgette’s system was a remarkable engineering achievement, carrying sewage away from the city to downstream discharge points. However, the system was designed for a city of roughly three million people.

Today Greater London has more than nine million residents, far beyond the capacity the original system was built for. More importantly, much of the Victorian drainage system uses what engineers call a combined sewer system. In this design, rainwater and wastewater share the same pipes. Under normal conditions, sewage from homes and businesses flows through the sewers to treatment plants such as Beckton in east London, where it is treated before being released back into the river.

The problem arises during heavy rain. When large volumes of stormwater rush into the sewers, flows can increase dramatically within a short period of time. If all of this water were forced toward treatment plants, pipes and pumping stations could become overwhelmed. In extreme cases, sewage could even back up into streets or buildings. To prevent this, the system includes overflow outlets along the river. When water levels rise too high, some of the mixed stormwater and sewage is discharged directly into the Thames. This mechanism is known as a Combined Sewer Overflow.

In the nineteenth century this was a sensible safety feature. But in a modern city with a much larger population and extensive paved surfaces, these overflows occur far more frequently. Before the construction of the Tideway Tunnel, there were dozens of overflow points along the Thames. During heavy rainfall events, large quantities of untreated wastewater could enter the river.

The engineering logic behind the Tideway Tunnel can be understood in three steps: interception, storage and treatment. Instead of allowing overflow pipes to discharge into the Thames, many of them are now connected to the new tunnel system. When the existing sewer network reaches capacity during heavy rain, excess flows are diverted into the Tideway Tunnel rather than into the river.

The tunnel itself acts as a vast underground storage reservoir. The system can hold about 1.6 million cubic metres of water, roughly equivalent to around 640 Olympic-sized swimming pools. During storms, the excess wastewater is temporarily stored inside the tunnel. Once the rainfall subsides and treatment plants regain spare capacity, the stored sewage is gradually pumped to Beckton for treatment.

The design also takes advantage of gravity. The tunnel slopes gradually from west to east, starting at depths of around 30 metres in west London and reaching more than 60 metres in parts of east London. This allows wastewater to flow naturally toward the lower end of the system before being pumped onward to the treatment works.

Construction began in 2016. Tunnel boring started in 2018, and the main tunnelling works were completed in 2022. The following years were spent connecting the new tunnel to existing infrastructure and testing the system. The full network became operational in February 2025, and the project was officially opened on 7 May 2025 by King Charles III.

The completed system is designed to reduce sewage overflows into the Thames by about 95 percent. For a river once described in the 1950s as “biologically dead”, this marks another important step in its long recovery.

The tunnel itself will never become a landmark. Most Londoners will never see it. Yet cities depend on precisely this kind of invisible infrastructure. Roads, power grids and water systems quietly support daily life without drawing attention. Tideway Tunnel sits deep underground, out of sight, but the improvement in river water quality will be visible and tangible. Residents walking along the riverbanks, and visitors coming to London, will gradually experience a cleaner Thames thanks to this unseen piece of engineering.