The Thames Tideway Tunnel will be a new "super sewer", the width of three London buses, which will run underground from west to east London.
It will be around 20 miles long - and will broadly follow the route of the River Thames, connecting up to 34 sewer overflows.
Sewage, which would otherwise spill into the River Thames, will then be captured in the new tunnel and transferred to a waste water plant in Beckton to be treated.
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The work, called Gift Horse, replaces the giant blue cockerel that previously sat on the plinth.
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