The portable metal detector; locating buried/hidden metallic stuff for over a century. Something to consider.
In some places around here.... that does nothing but aggravate a person.
The number of musket balls, cannon balls, this and that left over from the Civil War you could probably fill a wheelbarrow up in a few hours. I remember as a kid walking across a fresh tilled field, finding white powdery lumps here and there...clean them up and they were minniball bullets.