[Quix]

Oh, goodness. That too has come from a lifetime of reading a wide diversity of things.

You might try the following keywords:

COAL SEAMS ARTIFACTS

FORBIDDEN ARCHEOLOGY

STRANGE ARTIFACTS

STRANGE ARTIFACTS COAL SEAMS

PREHISTORY ARTIFACTS COAL SEAMS

PREHISTORY ARTIFACTS ENGINEERED

ARTIFACTS COAL SEAMS MODERN

ANCIENT TECHNOLOGY COAL SEAMS

and the like.

[demlosers]

One more: ooparts