A note from a French army officer accompanied the package. He told the scholars that the stone had been unearthed in an old fort near the town of Rosetta, thirty-five miles north of Alexandria. French soldiers were tearing down a ruined wall in the fort when they came upon the slab. The top right and left corners were missing, as was the bottom right corner. The soldiers had gone over the rest of the wall carefully in hopes of finding the missing pieces embedded in it, but with no luck.

—The Riddle of the Rosetta Stone,
James Cross Giblin
According to the passage, what caused the Rosetta Stone to be unearthed?