A hand-drawn map of Detroit was found in a house in Altamont, Ontario. Apparently it had been bought by the grandfather of the current owner of the house in the 1930s and remained there ever since. The map, in great shape after all these years, is the only one extant to depict Detroit under British occupation in the 1790s.
A very cool find, now on display at the University of Michigan William L. Clements Library.
The Detroit News: Centuries-old map of Detroit finds way to UM library