We’re working on our garden, in a new area. One part of the soil is darker. We figure the builders had a burn pile there. In addition to the rocks that are present in every garden (except Eden, unless God wanted Adam and Eve to sit. But I digress.), there are metal bits. Nails, mostly. A couple dozen of them, and wires. We’ve found probably close to 100 nails from this little patch of the garden, just a few square feet.
I wish the builders had been more careful, or cleaned up after burning the wood.