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London's Victorian sewer network is at bursting point. Its tunnels are regularly pushed past their limit, and each week the equivalent of 300 Olympic swimming pools full of raw sewage is released into the Thames, totalling 39 million tonnes every year. Now a huge engineering project is underway to massively expand the capital's capacity to deal with its own waste. Twenty miles long and seven metres wide, the enormous tunnel will be created directly beneath the River Thames by an army of 4,000 workers, and will capture this waste before it enters the river. The supersewer is the biggest upgrade to the sewage system for more than 150 years, but built 90 metres under London, it is also the biggest engineering challenge for a generation. In this three-part series, cameras follow workers across the capital over three years as they race to build the enormous tunnel on time and on budget.

The first episode follows the creation of the first stretch of the supersewer in east London. Lead engineer Emmanuel Costes has to build a massive 80-metre deep shaft that connects to the supersewer and make it watertight using a specialist technique called 'slipform'. It is a risky approach as it involves pouring concrete constantly for 15 days - and delays could spell financial disaster. The programme follows Steve Perry and his team at the treatment works in Beckton as they attempt to install the biggest pumps ever used in Britain to deal with sewage. The pumps are critical - with so much more sewage due to head to Beckton via the new supersewer, each one has to be capable of pumping three cubic metres of sewage per second. If they aren't installed correctly, millions of tonnes of the capital's waste will be stuck underground.

Originally built when the population was two million, the Victorian sewers now service nearly nine million Londoners. The episode shows how this growing population means the existing sewers need constant maintenance in order to work at full capacity. With huge residential developments being built across the city, a group of sewer technicians (aka flushers) are sent in to survey the old sewers and plan for new connections. The programme follows Emmanuel, Steve and the team, who are up against the clock to finish the first section of the new supersewer so that it can be put to work in time for the opening ceremony.