sentences:  The secret of Tarnsand was maintained until after the end of the war, when it was discovered that it was a mildly radioactive substance and the German mine detectors incorporated a simple Geiger counter.The Topfmine A had a flattened dome-shaped case with a raised flat circular pressure plate on the top surface surrounded by a circular shear groove.The case was normally made from pulped wood and cardboard mixed with tar for waterproofing.