Coal wash plants back in the 50s and 60s were unable to capture the minus quarter inch fine coal. Dozens of ponds full of fine coal were left behind in southern and central Illinois. In the 1980s, many of these ponds were reprocessed and the coal fines were recovered and sold.
Friends made millions doing this, but the value was not really from the recovered coal fines and instead was from the huge tax credits, expiring several years back.