The discovery of a mass grave at an east London Crossrail site was headline news last week. With around 4,000 skeletons interred there, the find was significant for its sheer size, but also because the most likely suspect is one of Europe’s all-time deadliest killers – the black death.

The 1665-6 plague outbreak in London, called the Great Plague, killed an estimated 100,000 people, around a quarter of the city's population. If the Crossrail skeletons do turn out to be from that time, the dig could end up being a groundbreaking one, both for historians, and for virologists working on the virus today.

When I call Crossrail’s lead archaeologist Jay Carver to ask more, he seems exceptionally calm for a man managing the hand excavation of several thousand bodies.

"The whole process is quite methodological," he tells me. "This particular discovery had a very obvious edge from it from the first days when we arrived at that level. It’s a case of very carefully lifting each individual skeleton out, defining its coffin edges as you go."

Finding a plague pit where you’re attempting to lay building foundations may at first sound a bit unfortunate, to put it mildly. Actually, though, the find is a lucky one.

"We know, right across London, there’s many, many, many former burial grounds and areas identified as specific burial grounds for plague victims. A lot of those are mapped .

"But of course, most of the time we try utterly to avoid disturbing them. It’s only this coincidence — the Crossrail ticket hall has to go in this particular location, to line up with the rest of the lines — that means we’ve really had to excavate the site."

Understandably, people can be a little sensitive about graves being disturbed: there are stories of tube station tunnels stopping dead where their builders rerouted them around burial sites. One historian even claims the Piccadilly line curves so much between Knightsbridge and South Kensington because 19th century tunnellers couldn’t get through the mass jumble of bone under Hyde Park.

"Coffins only came in in the 17th century, really. Prior to that you were more likely to be buried in a shroud."

This find is slightly more organised. Carver tells me that layout of the grave at Liverpool Street differs from what historical accounts, and the popular imagination, might lead us to expect. "We know that there was a great deal of panic during those plague years. Accounts of the time describe huge piles of corpses being loaded into pits."

Here, though, "everyone’s buried in a coffin. Which would suggest there was time to properly inter these people, as their relatives would no doubt expect. Perhaps London was better prepared to cope with something like the Great Plague – perhaps they were simply more able to cope with the number of dead".

The fact the skeletons have been interred in this way also helps date them. "Coffins only came in in the 17th century, really. Prior to that you were more likely to be buried in a shroud. We think this particular find dates from the latter half of the century."

There seem to have been a lot of these finds in London recently. Would this be the case in any European city, I ask Carver? Or is there something specific about London?

"No, not really," he replies. If anything, we have a morbid obsession with the events of 1665-6, precisely because it stands out as unusual. "The Great Plague of London has a place in the history books, particularly the school books, because it was really the last major outbreak of what was then described as plague.

"We’ve been looking for something like this: a very obvious, catastrophic grave, where multiple burials have been made on the same day. That points to the classic moment in the summer of 1665, where so many people were dying."

"Well, doctor, I've felt better." Image: Crossrail.

So what happens now? "The thing is to do the tests. This is the best candidate for a plague pit, but of course we don’t know precisely the date of these burials yet". It will take up to twelve months for carbon dating and the sequencing of the human bone to reveal the exact date, "but in terms of where we found these things, it should be then".

It’s not just historians who are excited about the find. The Crossrail archeologists also hand over samples — taken from the teeth of the dead – to scientists working on the history of disease, and the way that bacterial genomes have evolved. "Each new sample we give them helps determine how the bacteria developed over thousands of years," Carver explains. "And what potential mutations it may make in the future."

This means the skeletons found at Crossrail could help scientists who are working on the plague today. Those of us schooled in England tend to associate the plague with rats stowed on ships, jars of vinegar and a certain Monty Python sketch, but the disease hasn’t been eradicated. Outbreaks of plague have occurred in Africa, India and Madagascar in the recent past; several people contract it every year in the US. The large sample in this find could help determine whether this was the result of a single pathogen, or whether the bacteria has mutated.

In the meantime, though, the team has to clear the site. This is standard practice for historic burial grounds – "people don't accept mass machine excavation" – but it’s still a huge task. As we speak, the skeletons are currently being lifted up, bone by bone, shovelful of soil by shovelful of soil, into buckets, to be moved from where the ticket hall will stand.

By 2018, when Crossrail opens, this won't be a grave at all – and we may be a little closer to understanding one of the worst pandemics in historical memory.