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In just over a year’s time, where I am standing will be pitch black, devoid of human life and – on a bad day – full of human effluent. I am in London’s new super sewer, a monstrous concrete pipe that runs roughly along the course of the river Thames for 25 kilometres. It is designed to solve a problem that London and many other cities have been grappling with for decades: the discharge of raw sewage into rivers.

London’s current sewage system is creaking at the seams. It was built between 1859 and 1875 after the Great Stink of 1858. At the time, the city’s population was around 3 million. The visionary engineer Joseph Bazalgette designed a sewer to cope with 4.5 million people plus rainwater. Around 9 million now use it, the weather is wetter and London has been extensively concreted over, preventing rainwater from being absorbed by the ground.

The system can no longer cope. Around 60 times a year it overflows, dumping a total of 40 million tonnes of raw sewage per year into the Thames plus wet wipes, sanitary products, condoms and whatever else people see fit to flush down the loo.

“Our job is to build a sewer that will address that,” says , CEO of Tideway, the company behind the project. The answer they came up with was to build a gigantic overflow pipe deep underground, beneath the Victorian sewer, to intercept its overflows. The Tideway Tunnel is one of the biggest city sewer projects in the world, says Mitchell.

It won’t solve the problem completely: when the rain is really heavy there will still be discharges. But it will reduce their frequency to around three of four times a year, and the overflow will be predominantly rainwater. “The Bazalgette system gets full of undiluted sewage,” says Mitchell. “If it rains heavily, that gets channelled into the sewers, and the sewers get full. They then flush into the river. But it’s that first flush, which is pure sewage, which is the most damaging. We capture that.”

We kit ourselves out in high-vis clothing, hard hats and boots and head to the access shaft, a yawning concrete caldera about the diameter of a cooling tower and 50 metres deep. It has to be this big to get the huge boring machines down to where they do their boring stuff.

We clamber into the “VIP lift” – actually a metal cage attached to a crane – and are gently lowered to the bottom. From there we walk down the sewer itself to get a feel for the scale of this colossal engineering project. The circular tunnel is 7.2 metres across. It took eight years to build at a cost of £4.5 billion. Its total capacity is 1.6 million cubic metres.

There is no sewage in there now – the dirty work starts next year – and the tunnel is eerily beautiful, like smooth alabaster in the cold glow of the strip lights. “This is one of the most photogenic pieces of tunnel I’ve ever built,” says Mitchell. That is because there is a sinuous kink that is oddly pleasing to the eye. It wasn’t planned but they had to skirt around a boring machine that became stuck, doing a “turn and bury” manoeuvre to get it out of the way. The borer is now sealed behind the concrete tunnel wall and will be there forever.

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The tunnel slopes gently down from west to east, a few millimetres every metre. That enables the sewage to flow by gravity, no pumping required. “It doesn’t sound much but it’s enough to keep it moving,” says Mitchell.

This descent adds up to 55 metres along the length of the tunnel. When the sewage arrives at its destination, Beckton Sewage Treatment Works, it is 80 metres underground and has to be pumped back up. But the tunnel itself has no moving parts.

In around a year, the project will be complete. The lights will be removed, the access shafts capped and the tunnel plunged into lonely darkness for at least 120 years. It is possible that we are among the last people to set foot in it, says Mitchell. Maintenance inspections will be carried out by drone. “Technically we could go down if we have to,” he says. “But unless there’s a repair, which is highly unlikely, we’re unlikely to ever go down there again.”