We went over the whole footprint to check it was a firm base for our masonry, as we are not using concrete footings. It was all rock hard apart from two very localised mystery soft spots. These had to be dug out as they would not have provided a good bearing for our foundation walls.
This was the first - it was over two meters below finished ground level before we reached solid ground and it turned out to be a dogs grave (complete with a pile of bones!).