Archaeologists have confirmed the identity of the culprit behind London's Great Plague in a mass grave under a new tube station

Researchers just discovered the culprit behind the Great Plague of Britain which claimed a quarter of the country's population more than 350 years ago.

Excavation of Bedlam burial ground July 2015_204900

While excavating the plot for a new east-to-west tube station underneath Liverpool Street in London's Shoreditch neighbourhood, archaeologists uncovered the remains of more than 3,000 people in what they determined was a mass grave that potentially included victims of the Great Plague of 1665.

It is the UK's biggest archaeology project in history.

MOLA archaeologists examine 1665 Great Plague victim_244618

The positioning of the bodies and the estimated timing of their burials suggested to experts that they were victims of a sudden, untimely death.

Researchers have been running detailed tests on the remains for years, and finally got the results of a DNA test of some of the victims' teeth. The culprit? Yersinia pestis, a bacteria known to cause the plague.

Archaeologists say the mass grave, called the "Bedlam graveyard" after the nearby Bethlem ("Bedlam") Royal Hospital, was used for nearly 200 years beginning as early as 1569. In addition to serving as a public graveyard for Londoners who could not afford a costly church burial, the gravesite was likely also used as an overflow cemetery when existing sites had filled to capacity. This can happen during times of war or mass illness, such as a plague.

Crossrail, a subsidiary of Transport for London, has been deploying archaeologists to the site for more than a decade and has, along with the London Metropolitan Archive, enlisted a small group of volunteers from the area to compile a register of the names and backgrounds of the people buried there.

After discovering the remains, archaeologists and osteologists worked together to ensure that the teeth of the victims were carefully removed so they could be sent off for DNA analysis.

MOLA senior archaeologist Don Walker Henderson examines 1665 Great Plague victim_244613

"The best thing to sample for DNA is the teeth; they're like an isolated time capsule," lead bone researcher for the project Michael Henderson of the Museum of London Archaeology, told the BBC.

Scientists at Max Planck Institute identify 1665 Great Plague DNA_244611

And sure enough, in five of the 20 skeletons they tested from the Bedlam burial pit, the scientists found preserved DNA signatures distinctive of the plague bacteria Yersinia pestis.

Researchers still do not know why the Great Plague was the last major outbreak of plague in the UK, but they hope to find answers to these and other questions as they continue to study the genomes of the bacteria.

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