Catastrophe Planet — Keith Laumer

The Earth was in shambles after the final quake had leveled the cities. For Mal Irish the last hold on reality was the embossed gold coin he had taken from the pocket of the dead man - the man who with his last breath had told him of mastodons buried in ice and men who weren't human. Once in possession of the coin, Mal found himself on a mysterious quest which led him to discover even stranger things - the girl who spoke the language of another world, the city under the ocean floor, and the deadly little men who followed him. He was in the power of something beyond his understanding, and he meant to find out its source before it put him to its own unfathomable uses.