Summary: Mines are driven ever deeper because the world needs copper in the late phase of WWII. Shoed footprints of the size of three inches are found down there. Two engineers go down and discover one and half foot big humans swimming through concrete stone. They speculate that their bodies developed enormously dense under the huge pressure.
Review: A typical pulp fiction with an interesting trope of Hollow Earth providing an entertaining sensawonda like Old Mars stories. I found it interesting that the tiny humans are curious and dangerous explorers like us rather than dinosaurs. Not much of a plot or characterization.