In late 19th-century Leeds, the drinks were plentiful and strong—and, unfortunately, spiked with quite a bit of lead.

Archaeological Services WYAS’ ongoing excavation of a series of Georgian and Victorian period cellars has revealed an unexpected cache of centuries-old beer bottles some 600 strong, reports Andrew Hutchinson for the Yorkshire Evening Post. Originally thought to be some sort of ginger beer, the liquid that remains within actually appears to contain both alcohol and toxic metal contaminants, perhaps left by lead pipes feeding into the water brewers used to concoct their boozy batch.

The researchers made the discovery earlier this year at the site of the former Scarborough Castle Inn in the northern English city. Stacked in neat piles beneath the remnants of the 19th-century building’s stairs were several hundred bottles, some still corked and full of sloshing fluid.

Per the Drinks Business’ Phoebe French, the stash of bottles appears to hail from a mishmash of different breweries that were active in the 1880s, though most were emblazoned with the moniker “J.E. Richardson of Leeds.”

After popping open some of the brews, the team sent samples of the liquid out for analysis. Lab results, announced on Archaeological Services WYAS’ Facebook page last week, showed that the contents were about 3 percent alcohol by volume—the equivalent of a fairly mild English Session Ale, according to Eliot Routh of Vinepair.

Also present was a far less appetizing ingredient: lead, at a concentration of 0.13 milligrams per liter—well above the 0.015 milligrams per liter deemed safe by the Environmental Protection Agency. In this amount, the toxic metal may have caused the beer’s buyers to feel weak and sick, potentially causing irreversible damage to their internal organs.

“This beer would have been detrimental to [drinkers’ health],” the team writes on Facebook.

Though the source of the metal isn’t known, the researchers suspect it entered the brew by leaching out of lead-based pipes, which weren’t phased out of use until about a century after this batch of beer was likely concocted.

Vastint, the real estate company developing the property, will keep the bottles and feature them in an upcoming educational display, Archaeological Services WYAS Senior Project Manager David Williams tells Drinks Business.

“This excavation is giving us a great opportunity to uncover a part of Georgian and Victorian Leeds,” says Williams. “The results so far are giving a real insight to the daily lives of the former residents of Leeds during this period.”