They appear out of nowhere, cover the shining surface of silver coins with a milky coating and are regarded as spooky ghosts for collectors because they cannot be removed without damage: “Milky spots” are considered an unexplained numismatic mystery, troubling the joy of coin collecting. And in mid March 2017, the phenomenon even caused nationwide media interest in Germany because not a single private person was concerned, but a complete coin exhibition: the coin cabinet of the...