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.

In this second episode, construction begins in the heart of London. Two key connection shafts along the Thames are to be dug to enable the super-sewer to be built. In Battersea, a challenging construction technique called 'diaphragm-walling' is employed to build a massive 90-metre-deep shaft.

London's Victorian sewer network is at bursting point, and now a huge engineering project is underway to massively expand the capital's capacity to deal with its own waste.