When the bubonic plague broke out in London in 1665, it wasn’t the first time the disease terrified the city. But it was one of the deadliest outbreaks that the city had ever seen — and, possibly, the most infamous.

It’s long been thought that there might be victims of that crippling epidemic buried underneath London… under and alongside the trains people use every day.

This week, scientists confirmed there are.

* * *

The plague wiped out a great deal of 17th-century Britain. In London, it’s thought that fatalities reached up to 100,000, roughly one-fifth of the population. Popular accounts of the epidemic described its horror in detail. And they described the mass graves, known as ‘plague pits’, which are said to have gouged land across the city. Hundreds of bodies were said to have been hastily buried in these pits without coffins, care or ceremony.

Two hundred years later, London opened its first underground railway — the world’s first — in 1863. By the century’s close, a spider web of track linked stations as far as Shepherd’s Bush and Bank, Hammersmith and Mansion House.

If you look at these early routes, you’ll notice something odd: They don’t take the straightest, fastest line from point A to B. Instead, they meander. They curve. Almost like they’re avoiding something.

It didn’t take long to put two and two together. When engineers went to design these underground railroads, the story goes, they tried to avoid plague pits — either because they didn’t want to disturb the dead, or because the bodies were packed too thickly to bore through.

[Tube train lines] meander. They curve. Almost like they’re avoiding something.

Various resources hold that there are a number of places where engineers hit bodies… or tried to avoid them. Catharine Arnold, in her 2006 book Necropolis: London and Its Dead, writes: “At the spot where Brompton Road and Knightsbridge now meet, excavations for the Piccadilly Line between Knightsbridge and South Kensington Underground stations unearthed a pit so dense with human remains that it could not be tunnelled through. This is said to account for the curving nature of the track between the two stations.”

A crowd-sourced (but double-checked, its creator told me) map of London’s plague pits by the group Historic UK, later picked up by publications ranging from the Daily Mail to the Telegraph, mentions that a runaway lane for trains at Elephant & Castle junction is blocked off by a plague pit, while the Victoria Line drilled through a pit under Green Park.

“The Underground system passes through many burial grounds and plague pits,” says Peter Ackroyd in his 2012 book, London Under.

But did plague pits — or human burials at all — really affect the shape of London’s underground lines?

As it turns out, probably not. But the truth, in some ways, is even stranger… and even more morbid.

* * *

When I asked London’s transport historians about the plague pit-Tube connection, the overall response was a collective eye-roll. One after another told me that, in all of their record-scouring of Underground history, they’d never come across any mentions of plague pits.

“In all of the work I have done and am doing I have never encountered anything that suggested real or imagined plague pits influenced the construction of the London Underground,” Tube historian and author Mike Horne wrote in an e-mail.

Author Scott Wood found the same. “Contacting the Transport for London Corporate Archives, I was told that there are no specific references to plague pits in their records,” he wrote in his book London Urban Legends: The Corpse on the Tube and Other Stories. “I wanted to make sure — plague pits really are everywhere in London lore — so I went through the files on the planning and construction of the Victoria line and the Fleet line, which became the Jubilee line, under Green Park.” Wood, like everyone else, turned up empty.

Not only that. Aside from one well-known example at St Pancras railway station in the 1860s, experts say they hadn’t come across mentions of human remains — of any kind — in their research of the Underground’s early years. “Never found dead bodies at all,” says transport journalist and expert Christian Wolmar.

* * *

Experts were also quick to point out that we know why the subway lines curve, and it’s not because of plague pits. It’s because of cost.

The first routes, including the Circle, District and Metropolitan railways, were built using mainly ‘cut and cover’ construction. This meant digging a trench about 30ft wide by 20ft deep, bricking in walls and a roof, then shovelling everything over with about 6ft of topsoil. As a result, the railway companies had to purchase any private property affected by construction. In 1860, for example, the Metropolitan Railway bought and demolished some 1,000 homes for the line from King’s Cross to Farringdon.

Even when tunnelling was used more frequently — as it was later with the Central, Piccadilly, Bakerloo, Hammersmith & City and Northern lines — people believed that building collapses could occur if a tunnel was dug underneath. So the law still required that the railway companies buy any property they passed under.

As a result, whenever they could, the companies dug under publicly-owned roads.

It’s easy to see just how much of the Tube is explained this way by looking at a geographically accurate map, which you can see here. Looking it as an overlay on Google Maps, like with this version, makes it easier to see the roads.

This alone deflates a lot of the anecdotes. That bit between South Kensington and Knightsbridge, for example, suddenly looks a lot less suspicious: it’s following Brompton Road, not skirting resting places for plague victims.

Since most mentions of the Piccadilly Line-plague pit connection reference Catharine Arnold’s book Necropolis, I asked her where she’d found it.

“It’s anecdotal,” she told me. “I got that from a then-boyfriend.” She didn’t know where he’d heard it from.

In any case, it would be strange for the Piccadilly Line to have to go around a plague pit, transport experts say. As one of the later railways, it was dug deeper than those earlier cut-and-cover lines, running between about 40 and 80ft below the surface. That’s far below the level of any supposed plague burials.

And even if there were burials near an underground line, it’s unlikely that that would have affected their design, says David Long, author of London Underground: Architecture, Design and History.

“The notion of diverting a line because of that seems far-fetched,” he says. “I think that it would just plough straight through it.”

* * *

According to the story of the Green Park plague pit, that’s what happened with the Victoria Line.

But when I searched through numerous newspaper articles written about the route’s construction in the 1960s, I couldn’t find a single mention of human remains — despite how much such a story would have helped sell tabloids.

Later, in the Museum of Transport library, researcher Caroline Warhurst and I searched through books for any references at all.

We found nothing about plague pits.

There were a few mentions of Underground construction stumbling upon human remains, even if not plague victims, but they were nearly all found rather recently. In 1992, 160 skeletons, mainly of destitute women and babies, were removed from a crowded 19th-century burial ground at Redcross Way during tunnelling for the Jubilee Line extension. Two years later, 21 years of digging for the Jubilee’s new Stratford depot wrapped up at the burial ground of the Abbey of St Mary’s, Stratford Langthorne; 647 graves were excavated.

But what about the Underground’s earlier days, over a full century earlier? There was only one tantalisingly short detail.

It turned up in Alan Jackson’s London’s Metropolitan Railway, a 328-page tome detailing seemingly every debate, cost and parliamentary bill regarding the Underground’s genesis. But it only had this to say about when the line from Paddington to King’s Cross hit remains in 1862:

“Much of the excavation was made through the dust and debris of past ages, which in some places lay in a stratum up to 24ft deep. Human remains were encountered, payment being made to the London Necropolis Company for their removal and reburial at Brookwood, whither the bones of those who had never known railways travelled by train.”

Scouring 19th-century newspapers led to more examples.

At a Commissioners of Sewers meeting in October 1865, the Standard reported, someone had written to the North London Railway “complaining of the manner in which certain human remains found in excavating the ground for one of their stations in the City… were alleged to have been treated.”

The company secretary looked into it. He said that, yes, bones were found — and they were stuffed into one of the railway arches for safekeeping while the company decided what to do with them.

The commissioners’ clerk had a suggestion for North London Railway as what to do with the bodies: he told them that recently, human remains had been found in West Street, Smithfield during the construction of a railway. He said that “they were carefully collected, put in coffins, and conveyed to the City of London Cemetery at Ilford.”

Even Westminster Abbey’s burial ground was affected by the construction of a train line.

“Backed by an Act of Parliament, nothing may be considered safe, above ground or beneath, if a railway is required,” wrote the Morning Post in September 1866. “A striking illustration of this fact is just now occurring before Westminster Abbey, a portion of whose burial ground the new railway undermines. The passer by may notice that the whole northern side of the abbey churchyard is enclosed by a lofty hoarding, cutting off a large slice of the burial ground. Here the excavations for the new underground railway are in progress; and the human remains laid in consecrated earth, and vainly supposed to have been deposited till the day of doom, are being removed, coffined and uncoffined, to the necropolis at Woking — a more fitting and more safe final resting-place.”

Yes, bones were found — and they were stuffed into one of the railway arches for safekeeping while the company decided what to do with them.

There were other hints that this wasn’t an entirely infrequent occurrence — and that it was a trend that spanned a couple decades. In 1884, Isabella Gladstone for Pall Mall Gazette wrote that the railway companies “even endeavour to appropriate the square-gardens and the small burial-grounds (consecrated or unconsecrated) which they find conveniently situated for their purpose.”

“There is scarcely a railway line which does not run over a few graveyards,” she wrote.

Later, Roy Stephenson, the Museum of London’s head of archaeology, mentioned to me that he’d recently come upon a memorial he’d never seen before: an unassuming monument tucked into a wall on Cloak Lane, an area near St Paul’s Cathedral. The memorial simply reads: “Sacred to the memory of the dead interred in the ancient church & churchyard of St John the Baptist upon Walbrook during four centuries. The formation of the District Railway having necessitated the destruction of the greater part of the churchyard all the human remains contained therein were carefully collected and reinterred in a vault beneath this monument AD 1884.”

Like many other churches in the City of London, St John the Baptist upon Walbrook burned down in the fire of 1666. Only its burial ground remained — until it was cut through by the District line.

I emailed Warhurst at the library. “We don’t have a note of that memorial here, and it isn’t mentioned in the relevant published histories covering that bit of the story,” she wrote back. “We’ll add that to our records.”

* * *

Transport workers hitting human remains in London makes sense, given how many unmarked burials there are — even if such incidents only turn up rarely in historical accounts of the Underground being built.

“Just about every green space you find in the city was probably a form of a burial ground,” says Jay Carver, lead archaeologist for Crossrail, London’s high-speed, deep-tunnel railway currently under construction. “A lot of those will still have human remains below ground. But they’re not marked.”

Some of that is from former burial grounds that lost their churches in the 1666 fire. More often, though, it’s because of a law passed in 1852 to close the city’s cemeteries.

By the early 19th Century, London’s churchyards were becoming too crowded; coffins sometimes jutted out of grounds and the air filled with a stench blamed for causing lethargy, fever, even cholera. Closed in the 1850s, many were re-landscaped and cleared of gravestones — but not always of human remains.

But few if any of those burial grounds — and none that the railways seem to have hit — were ‘plague pits’.

That’s partly because plague pits themselves are rarer than people think. As long as land was available, plague victims were buried in cemeteries: usually in churchyards, along with those who died of other causes.

“The plague is a terrible experience for Londoners, so in some ways they cling on to things that they’re used to, that give them stability and comfort,” says University of London historian Vanessa Harding. “And one of those things is, as far as possible, people should be buried properly.”

But what of sites for only plague victims, ones outside a church’s formal burial ground? Harding says there are “only a handful.”

“People think that every time something is excavated and there are bodies, and they didn’t know they were going to be there, it has to be a plague pit,” she says. “The first point is that they were not scattered all over the greater London area: as far as emergency plague burials in pits, it was quite closely around the City and West End. The second is that most of them were known where they were. There were contemporary records of them. I don’t think that there are a great many that nobody knew about.”

Meanwhile, several of the most famous plague pits of lore, including those beneath Green Park and at Brompton Oratory, have no historical basis: aside from rumour, there’s no evidence — archaeological or otherwise — that they ever existed.

It’s unclear how, in the popular imagination, so many have passed from legend into fact. But Necropolis author Catharine Arnold’s admission that she heard about the Piccadilly Line plague pit in passing might provide a clue. Once it’s in print, a rumour becomes real: several other people I spoke with pointed me to her book as a source.

There is scarcely a railway line which does not run over a few graveyards

The legend has only grown. Even in June 2016, when a passenger captured a video of an “apparition” seen on the Piccadilly Line between Knightsbridge and South Kensington, the tabloid Express ran a story about how paranormal investigators thought it could be the “spirit of a victim of the Black Death.” That was treated a little sceptically. But no such scepticism applied to the basis for the ghost story: that the tunnel curved around the Brompton Oratory plague pit at all. “Plague pits actually influenced the direction of many London Underground tunnels, carved out 150 years ago, with lines often having to curve around mass burial grounds such as this one,” the story says confidently.

But the blame can’t only be laid on contemporary writers. Some of the confusion also comes from reliance on writers of the time. Daniel Defoe’s drama Journal of a Plague Year, for example, lists a number of plague pits. But Defoe was a child at the time of the 1665 event, and though he billed his book as being a victim’s eyewitness account, it is now generally considered to be a work of historical fiction.

Carver pointed out how chroniclers of the Black Death of 1347-8 suggested that 50,000 plague victims were buried at Charterhouse Square. Two years ago, his team excavated there. “Clearly it’s a massive exaggeration, from what we’ve seen,” Carver says. “We’re thinking more like 5,000.”

But that’s not to say plague burials never happened — or that they haven’t ever involved London’s underground lines.

It was just much more recently than people think.

* * *

Because the high-speed Crossrail tunnel is so deep, it avoids most of London’s archaeology. The exceptions are where the tunnel enters the ground, plus the shafts and the stations.

Crossrail archaeologist Carver says these exceptions added up to 40 spots in the city, all of which were investigated. Only two — Charterhouse Square in 2013 and, in 2015, Liverpool Street Station — turned up human remains.

Neither was a surprise. At Charterhouse Square, for example, labourers had turned up bones long before Crossrail’s dig: during work on the sewage system in 1834 and again in 1861; when building a railway at West Smithfield in 1865 (presumably the incident in the Standard article which the commissioners’ clerk referred to); and again in 1885.

“When it comes to human remains, it’s illegal to disturb them… without a licence,” Carver says. “So when it comes to burial grounds, the first thing you have to do, when planning a line like Crossrail, is plot all the burial grounds we know about, historically — and try and avoid them.”

Still, he added, “It’s a minor consideration in terms of building a railway like Crossrail, because the most important thing is accessibility. You can’t put a station where no one wants one.”

Planners for High Speed 2, another high-speed rail network, are finding that now. Part of the project will include redevelopment of Euston Station, which calls for the excavation of St James’s Gardens, an 18th-century cemetery-turned-park. “It’s an important heritage site. It’s a shame if the engineers can’t come up with an alternative,” Carver says. “But you can guarantee that if they decide it’s the only option, it’ll be because it is.”

The benefit, meanwhile, is that the projects allow for rare archaeological excavations in the heart of the city.

Like the Charterhouse dig three years ago.

Finding plague victims there wasn’t a surprise. By 1346, the Black Death had begun its sweep of Europe. Historical records show that the government of London, wanting to prepare, pre-emptively purchased and set aside land in the area to use for burial when it arrived. Two years later, it did, making the site one of the “handful” of purpose-built emergency burial sites that Harding and others referred to.

An extensive dig at East Smithfield in the 1980s excavated the remains of some 759 people — a fraction of the burial ground’s estimated 2,400 Black Death victims. In 2011, DNA tests confirmed they had bubonic plague.

Just as interesting, though, was the care with which they were buried. Far from the idea of people being heaped in, the coffins were placed in neatly dug trenches, one some 410ft long.

So when the Crossrail team came to do a test excavation in 2013 a stone’s throw away, at West Smithfield, they knew what they might expect: the other (and older) of the two emergency burial grounds records showed to be in the area.

And they were correct. Opening up a shaft just 18ft wide, they uncovered 25 skeletons. Testing a sample dated them to three different burial groups: 1348-9, 1360 and 1430. All three phases were confirmed to have samples of the bacteria that caused plague. Again, they were neatly buried.

The Crossrail excavation was the first time that evidence of London’s second emergency burial pit was firmly established. And — despite the rumours of them being located, helter-skelter, across the city — these are the only two purpose-built ‘plague pits’ from the Black Death of 1348 ever to have been confirmed.

The Crossrail digging didn’t stop there.

The second major Crossrail dig, for the ticket hall for Liverpool Street Station in summer 2015, has been tied to plague, too. But, for historians, in an even more exciting way.

That’s because while both the Black Death of 1348 and Great Plague of 1665 were devastating — the Black Death is thought to have wiped out up to two-fifths of the London population, the Great Plague up to one-fifth — the Great Plague is the one that most people associate with chaos, terror and plague pits.

And yet those Great Plague pits remain elusive. In fact, no archaeological discovery of the 1665 epidemic’s victims had been confirmed in London before.

Until Crossrail’s dig in 2015. Some 3,500 skeletons were unearthed at the site of the Bedlam cemetery, a 16th- and 17th-century burial ground now under Liverpool Street. About 42 of the individuals, though, were different from the rest. Rather than having been buried separately, they were in a single mass grave.

Still, as with the Smithfield dead, they’d been buried in coffins and rows — even if the coffins having rotted over time made them now look like they’d been heaped together.

But they were stacked up to four deep, without any soil between them. In other words, they weren’t grotesquely thrown in… but they were buried in haste, and likely on the same day.

This week, the DNA test results from the 'plague pit' burials at the coming Crossrail line (and near where the Central, Circle, Hammersmith & City and Metropolitan lines converge on Liverpool Street) were announced. They were positive for the bacteria that causes bubonic plague.

That makes those skeletons the only plague victims from the 1665 epidemic to be confirmed with DNA in the whole of the UK.

“Part of the payback for the fact that we couldn’t avoid it is we do get really important research,” Carver says. “It’s the first archaeological sample of any burial ground in the city of this date.”

In both cases, when they began digging, Crossrail was well aware of what they would find — and were able to excavate accordingly.

But that hasn’t always been the case.

* * *

When I asked archaeologists at the Museum of London about times when London’s transport construction hit — or avoided — human remains in London, they paused.

“The Eurostar extension,” blurted out Jelena Bekvalac, the museum’s curator of human osteology. “Oh,” groaned the two others, Roy Stephenson and Rebecca Redfern, in unison. “That was ghastly!”

In 2002, plans were underway to extend and renovate St Pancras station, including building a new terminal that could accommodate the new, 1300ft-long trains that would speed through the Channel Tunnel. The work would cut into the cemetery of St Pancras Old Church, just to the north of the station, affecting some 40,000 square feet of burial ground.

“So traumatising,” said Stephenson, who had been one of the archaeologists at the excavation. “It started off well. You had osteologists on site. But it basically took too long, so they invoked the full tenant of the act [of Parliament, which was required to build] and removed the human remains in a more — mechanical fashion.”

Mechanical fashion? “Yeah. I think there was a conveyor belt involved.”

“There was. And one of those pile drivers that you dig down, and just digs through coffins and plates and people,” Bekvalac said.

The archaeologists had been under (now expired) confidentiality agreements, so only a couple of stories — including one in the Evening Standard and one on the BBC — seem to have made it to press, along with an official (and seemingly anaesthetised) book about the excavation.

Meanwhile, according to the BBC, the company had been able to avoid the “normal special permission required when building work disturbs a cemetery.”

The burial grounds of St Pancras Old Church had been disturbed before. Just a decade after the cemetery’s 1854 closure, the Midland Railway cut across them. At the time, this caused a great deal of controversy — making it the only example of human remains being moved for London’s railways that most historians I spoke with had heard of. Like many other London cemeteries, the cemetery was infamously overstuffed. The work was messy, done at night, involving hacking through coffins with spade and pickaxe.

One of those watching was the writer Thomas Hardy, who worked as an overseer for the Clerk of Works. He wrote a poem inspired by the scene which memorably began: “We late-lamented, resting here, Are mixed to human jam, And each to each exclaims in fear, ‘I know not which I am!’”

An estimated total of 7,403 bodies were exhumed at the time.

Which meant that, when the St Pancras project began in 2002, those involved thought the burials had already been cleared.

“People assumed that it had already been destroyed, and when it came to construction, it wasn’t. So it was catch-up to get the archaeological team in place,” says Carver, who was also one of the archaeologists on the site. “It was a lot of frantic negotiation — with a lack of time to do it.”

We late-lamented, resting here, Are mixed to human jam, And each to each exclaims in fear, ‘I know not which I am!’

At one point, to speed everything up, archaeological excavation of the site was halted completely while machines dug through the soil.

That event is part of why, Carver said, he never makes assumptions.

* * *

On a recent afternoon, I walked past the gleaming glass-and-iron leviathan of St Pancras International terminal. About 300 yards up the road from the bustle of people and traffic was a small green hillock, walled in from the street, its ground starting some 3 or 4ft above the street level. But it wasn’t a natural hill; its height was from the number of people buried there.

This, of course, was St Pancras Old Church. To the naked eye, its burial grounds appear like any other. Old tombstones dot the grass; trees arch into the sky.

But it’s not. Tombstones cluster at the base of an ash tree: they were among those moved during the Midland Railway’s construction, memorialised with the planting of the tree — now known as the ‘Hardy Tree’. In a nearby corner, 31 tombs are lined up in a row, seemingly moved from their original spot. Next to them are 16 rows of flat grave slabs, grown over with weeds, inscriptions mostly illegible. Brick walls cut off the grounds at clinical angles. Just beyond, railway tracks start their run to Europe.

In the end, the popular urban legend doesn’t hold up. The people who were buried here didn’t die of plague — though many likely died of other scourges like cholera, or smallpox. And their burial didn’t alter the shape of London’s railway lines.

But they might have found the truth — that their bones would be moved in the great, and ongoing, construction of London’s elaborate transport networks — even stranger.

And those who remain in their original resting place don’t hear how, every few minutes, a train rattles past, drowning out the silence.

This story is a part of BBC Britain – a series focused on exploring this extraordinary island, one story at a time. Readers outside of the UK can see every BBC Britain story by heading to the Britain homepage; you also can see our latest stories by following us on Facebook and Twitter.