Our house is supposed to have a coal mine 75' below us according to NCB records. In fact when our water borehole was drilled it found it at about 46'.
We have several depressions in the fields around us caused by older mines which the NCB have no records of. Some of them have old mine trucks and rails in them still where they were abandoned.

So, in short, there are a lot of mine workings down there that have no records left about them.