Thirty-nine million tonnes of raw sewage are pumped into the Thames each year because of London’s outdated sewer system.

Built in the 1860s, the existing infrastructure still works perfectly but it cannot keep up with London’s huge (and growing) population.

Engineer Sir Joseph Bazalgette designed the sewers with population growth in mind; he anticipated the two million residents doubling to four million – but that number has since doubled again, and is constantly rising.

Because it deals with both sewage and drains, the Victorian system was designed to release overflows (usually of storm water) into the Thames, instead of flooding roads and people’s homes.

This initially happened twice a year, but now overflows happen on average once a week – about 750,000 tonnes each time.

The Thames Tideway Tunnel is a £4.2 billion project to build a system of huge pipes, dubbed a “super sewer”, underneath London with the aim of modernising the capital’s sewage system and making the river cleaner.

The extra capacity from the 25km (15 miles) of tunnels, running from Acton in West London to Abbey Mills in the east, should mean the system only discharges four times a year.

This will make the Thames much cleaner, which would be better for wildlife and the environment, with Tideway bosses hopeful the work could bring the river back into greater use by Londoners.

Work has started on the largest section, with tunnel boring machine Millicent (named after the suffragette Millicent Fawcett) beginning to head westwards from Nine Elms, Battersea, in November.

Millicent simultaneously digs the tunnel and concretes the new “walls” it creates.

Other machines will dig from the rest of the 24 sites, including Annie (after Annie Maunder, the first female astronomer to work at the Greenwich observatory), which will begin in Greenwich in 2019.

Work on the whole project is expected to finish in 2023.

Some facts about the tunnel:

The tunnels will be between 35m and 65m below ground – 35m is the minimum height of a “high-rise” building

At 25km long, the main tunnel would take a car nearly 30 minutes to drive down at 40mph

The tunnel will be 7.5m wide, about the same as a standard single-carriageway road

Currently, the amount of untreated sewage that spills into the Thames each year is the equivalent of eight billion toilets flushing

The Great Stink

Sir Joseph Bazalgette was asked to design the existing sewers after The Great Stink – the summer of 1858 when exceptionally hot weather made the stench from the Thames unbearable.

Even before then, three cholera outbreaks were blamed on the disgusting pollution in the river, and Bazalgette’s work is thought to have saved more lives than that of any other Victorian official.

A medieval mystery

During Tideway’s work in Bermondsey, workers found a 500-year-old skeleton of a man wearing leather boots.

Archaeologists think his strong shoes suggest he worked on the river, perhaps as a fisherman, and the strange position in which he was found indicates he died either after falling in or being pushed.

Experts found evidence of blunt force trauma to his skull, a broken (but subsequently healed) nose and deep grooves in his teeth – possibly from passing a rope through them.