Inside the Colossal Super-Sewer Being Built Under Your Feet

London has long been obsessed with the effects of its own shit. We don't know what Roman Londoners did with theirs, but we can be fairly certain that cesspits gave way to crude underground drainage systems in the 13th century, which weren't remotely sufficient to contain the effluence flowing into the Thames.

Things would only get fouler. An Act of 1531 saw a group of London worthies grouped together as "commissioners of the sewers" – a title that gave the power to build nine new sewers all over the city, from Westminster to Poplar. They weren't enough for long, with hundreds more constructed until the Victorians properly professionalised London's waste after an 1848 survey revealed a dilapidated, dangerous system on the verge of collapse. One report records the bricks of a Mayfair sewer so rotten "you could have scooped [them] out with a spoon".

The resulting subterranean masterpiece was the vision of Joseph Bazalgette, chief engineer of the Metropolitan Boards of Works. His ornate network of the new and ancient took the collected awfulness and diverted it away from the Thames and out beyond the unspoiled fringes of the city limits. From there, London has squatted in blissful ignorance for the past century-and-a-half. But no good thing remains unchanged, and here we are: at another point of crises for the Byzantine city of filth beneath our feet.

You'd be forgiven for not having heard of the Thames Tideway Tunnel, the UK's first "super sewer". Unlike whatever sketchily-financed ornamental skyscraper is in the news this week, it's not something to be spotted with the naked eye – but its scale and cost are immense enough to propel any proper conception of it just out of most people's imaginative reach.

The number are dizzying: 25km of newly burrowed tunnel underneath the river, spread across 24 sites from the old pumping station at Abbey Mills in the east, all the way to Acton in the west. At 7.2 metres wide and 66 metres deep, it has the capacity to deal with 1.6M cubic metres of shit, slime, piss, puke, face wipes and cooking oil. A snip at £4.2 billion, controversially funded by jacked up Thames Water bills and on track for completion in 2024.

The fundamental problem with Bazalgette's sewers is that they were designed for a London of 2 million people. By the end of this year, most estimates point to a population of 8.8 million. By 2160, that number will stand at 16 million, providing civilisation makes it that far.

That's a lot of extra filth to funnel into creaking pipes and dilapidated alleys. It's why mention of the "fatberg" conjures up very tangible, very recent memories. It’s why the "concreteberg" exists, and why tons of excess waste water are currently flowing into the Thames. It also explains why the Tideway is one of the most ambitious infrastructure projects in London's history, and the urgency of its timetabling.

It's not quite "urgency" I feel, sitting in the lobby of Tideway's Fulham site on a sweltering summer afternoon. "Trepidation", "cowardice" and "nervous excitement" seem more appropriate. After weeks of back and forth, they've agreed to let me journey down into the bowels of one of the holes far enough into construction to permit a worthwhile descent.

Thankfully, my guide, Peter Layton, is a deeply reassuring presence. The Tideway site manager has spent the majority of his career underground. Despite – or maybe because of – this, he's an almost boundlessly cheerful figure.

Tunnels are Peter's thing and have been for over 20 years, he confides. It's a good professional life all told, lacking too many dull moments and containing regular flashes of fulfilment, with a personal highlights reel that includes the building of the DLR. It also gives plenty of fodder for pub conversations, he tells me, "which seem to entertain my friends who spend their lives in offices. They certainly ask more questions about my work than they do of the blokes working in insurance."

Before heading down, we breeze through a fairly exhaustive list of don'ts, then don the full array of protective gear, in all of its body heat absorbing, hi-vis glory. I'm given a pack, just in case of any unspecified emergency, comprised of a gas mark and what appears to be a flare. Reassuringly, we’re told that no one has yet had to use it, though "there’s always a first time for everything".

Peter answers my bumbling layman's questions with good grace as we move through the yard towards our descent. There are over 30 workers on shift at any one time, he explains, "working night and day, above and below ground". Another hundred or so staff the offices, with several others serving the giant tunnel boring machine that cuts its way through the crush of soil, clay and debris deep underground.

Walking through the yard means a whistle-stop tour of the acoustic shed, one of the first and most crucial things to be built on the site, allowing for 24-hour working conditions, cutting out any nuisance noise for the local, blissfully unaware neighbours. A 40-ton gantry crane drops supplies down, while the day of my visit also sees the installation of the last section of conveyor belt that allows for the tunnelling machine to move onto its next section, further west.

It's difficult to convey the feeling of awe and dread, when confronted with the yawning mouth of the hole. Tempting as it is to reach for the language of science fiction, it doesn't really come close. There's empathy for the Victorian journalist, confronted with the scale of the then-new Abbey Mills treatment plant, who believed himself to be "in the very jaws of peril, in the gorge of the valley of the shadow of death, [close to] the filthiest mess in Europe, ready to leap out like a black panther".

"In 120 years, this will still be working to capacity," Peter shouts over the roar of the lift as we arc our way down. "Who knows if anyone will still be living in London then, but this will all be here. It's been done in layers as you come down. There's a 3,000 cubic-metre slab on concrete just there, to stop us being a buoyant shaft," he explains as we slap to a halt, 60 metres below ground.

There were all sorts of questions I'd meant to ask. About the alternative measures that critics say would have been a better use of resources, or more useful feats of engineering. I'd asked a few already critical experts what they thought of Tideway and their answers tended towards ambiguity, touching on important, if perplexing, engineering matters. None were quite as vociferous as Professor Chris Binnie, who assessed the claims for a new super sewer in the mid-2000s and branded Tideway a "stupendous waste of money for very little benefit" in 2016, while claiming that equally significant change could be achieved at a fraction of the cost and disruption.

But the criticism is now redundant: this is how our city's waste is to be treated, for the next century; to argue retroactively for one means of disposal or the next is futile.

These thoughts start to melt as we dive further and deeper into the heart of things. We bump into a cosmopolitan crew of workers, from absorbed-looking drivers in the control room, to random apparitions engaged in mysterious bits of seemingly convoluted labour.

Soon enough, Peter draws our attention to a small steel room with two seats and doors of preposterously thick steel. He explains before the question arrives that it's a de facto panic room in case something cataclysmic were to happen and the outside defences broke. "You could stay in there for about two days, waiting for help to come. But you don't need to worry – it's only the very last precaution," he adds with a grin.

Why exactly is a trip to Tideway so disconcerting? It's nothing to do with the professionalism of the work, its objective necessity or the skill of those carrying it out both over and under ground. London is usually fairly remorseless in trumpeting its colossal infrastructure projects, from HS2 to the reopening / rebranding of the old East London Line in 2011.

Tideway is different. It's a set of fears both abstract and horrifyingly concrete; a simultaneous trip into the future and the past, right into the heart of our most putrid, intimate secrets. The grandest expression of the everyday horror and oddness of metropolitan life, of all the things tucked away and repressed out of sight and mind.

Blinking back into the sunlight, we make our way back to the air conditioned normality of the main office. Later, on the top deck of the bus home, thoughts drift to one of the stories I'd read in preparation for the day's descent. For centuries, some of London’'s most despised outcasts dealt in the detritus of its sewers. They'd search for relics, metals, scraps of soap to sell or reuse, often for little or no gain. The life of a "tosher" was nasty, brutish and short, but it was a way of life.

In December of 2018, a 500-year-old skeleton of a man in knee-high leather boots was found face down near a Tideway site in Bermondsey, buried in layers of ancient ooze and scum. His footwear suggested to experts a scavenger who died a dismal and accidental death. Forensic analysis of his arthritis-ridden bones spoke of a short, painful existence.

There's probably a message somewhere in the pitiful discovery, but the only thought blowing through my mind is how good the fresh air feels, back in the city of the living.

@Ffranciscodgf / @christopherbethell