For nearly two decades leading up to the signing of the Declaration of Independence Benjamin Franklin lived in London in a house at 36 Craven Street. In 1776, Franklin left his English home to come back to America. More than 200 years later, 15 bodies were found in the basement, buried in a secret, windowless room beneath the garden.

In 1998, conservationists were doing repairs on 36 Craven, looking to turn Franklin’s old haunt into a museum. “From a one metre wide, one metre deep pit, over 1200 pieces of bone were retrieved”—remnants of more than a dozen bodies, says Benjamin Franklin House. Six were children. Forensic investigations showed that the bones dated to Franklin’s day.

Franklin was a noted revolutionary and powerful freemason—the Grand Master of Masons of Pennsylvania—so it’s easy to wonder what dark secrets Franklin may have hidden in his basement chamber. But the truth, it turns out, isn’t quite so dark.

“The most plausible explanation is not mass murder, but an anatomy school run by Benjamin Franklin’s young friend and protege, William Hewson,” said the Guardian in 2003.

Still, in Franklin’s time, anatomy lessons were a dark, ethically ambiguous business. Mental Floss:

Anatomy was still in its infancy, but the day’s social and ethical mores frowned upon it… A steady supply of human bodies was hard to come by legally, so Hewson, Hunter, and the field’s other pioneers had to turn to grave robbing — either paying professional “resurrection men” to procure cadavers or digging them up themselves — to get their hands on specimens. Researchers think that 36 Craven was an irresistible spot for Hewson to establish his own anatomy lab. The tenant was a trusted friend, the landlady was his mother-in-law, and he was flanked by convenient sources for corpses. Bodies could be smuggled from graveyards and delivered to the wharf at one end of the street, or snatched from the gallows at the other end. When he was done with them, Hewson simply buried whatever was left of the bodies in the basement, rather than sneak them out for disposal elsewhere and risk getting caught and prosecuted for dissection and grave robbing.

Franklin was probably aware of the illegal studies going on in his building, says the Benjamin Franklin House, but it’s doubtful the he was involved himself. Still, we can’t imagine that, curious man that he was, he didn’t sneak down and check out the proceedings at least once or twice.

More from Smithsonian.com:

Ben Franklin: Patriot, Foodie

Did Benjamin Franklin Invent Daylight Savings Time?