The London Underground network's new Crossrail line, under construction since 2009, is the most expensive infrastructure project in Europe. Twenty-four billion dollars worth of high-speed rail spanning 73 miles and 37 stations across the city. Lots of spending and digging, obviously. An excavation crew even managed to dig up a mass grave filled with pottery and human rib cages.

Testing has confirmed that skulls discovered at a few Crossrail construction sites over the past year carry traces of Yersinia pestis, the bacteria linked to the Black Death that plagued medieval Europe, first infecting Britain in 1348. Historians had known that thousands of Black Death casualties were buried in or around London between 1348 and 1350. But they were uncertain of the mass grave's location until last March, when digging crews discovered twenty-five corpses around Charterhouse Square in central London, buried eight feet below Farringdon Road.

Scientists are examining pottery shards at the burial sites to learn more precisely when the graves were dug. Apart from cause of death, the human remains also yield clues to the general nutrition of the deceased, most of them poor. All of them died without Oyster cards, which are relatively expensive.

Jay Carver, the Crossrail project's lead archaeologist, expects that his team will unearth "a much bigger mass burial trench."

Last year, they said the remains probably belonged to victims of the plague, which killed about a third of England's population following its outbreak in 1348. Limited records suggest up to 50,000 victims were buried in the cemetery in London's Farringdon district, one of two emergency burial sites.

Nick Elsden, a project manager at the Museum of London Archaeology, adds that "we don't know just how many skeletons are out there."

If you see something, say something.