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 it will capture this city's waste before it hits the river. The super sewer is the biggest upgrade to the sewage system for more than 150 years but built 90m under London, and it's also the biggest engineering challenge for a generation. In a major new three-part series, Five Billion Pound Super Sewer puts cameras at the heart of the action, following workers across the capital for the past three years as they race to build the enormous tunnel on time and on budget.