For nearly two decades leading up to the signing of the Declaration of Independence, Benjamin Franklin was a boarder in a house at 36 Craven Street in London. The future founding father left his English home and returned to America in 1775. Two centuries later, bones from more than a dozen bodies were found in the basement, where they had been buried in a mysterious, windowless room beneath the garden.

The skeletons had gone unnoticed until the 1990s,…