Wikipedia
The Topfmines were a series of German circular minimum metal anti-tank blast mines that entered service with the German army in 1944, during the Second World War.
The mines used a case made of compressed wood-pulp, cardboard and tar along with glass plugs and components designed to be undetectable by Allied mine detectors. Often the only metallic part of the mines was the detonator.
To enable the mines to be found by friendly forces, the mines were painted with a black sandy substance called ( camouflage sand). Allied forces found that although the mines were undetectable by Allied mine detectors, German mine detectors could find the mines when they had been marked with Tarnsand. 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.