Kanaalmunten
Despite the name, Kanaalmunten (literally, “canal coins”) can come from any recognized body of water: canals, rivers, ponds, bays, streams, oceans… even an abandoned swimming pool, if the conditions are right. Said conditions are: somebody has to accidentally lose a metal coin in the water, and the coin has to remain there undisturbed for at least a year and a day. When that happens, the coin becomes a Kanaalmunten.