On July 9th Walking Borders, an aid group, reported that 300 migrants travelling on a flotilla of boats from Senegal towards the Canary Islands, a Spanish archipelago, had disappeared. That number was a well-informed guess. Unless the migrants are rescued or their bodies found, they will be classified as “missing". Events like this are common, both at sea and on land. On June 14th an overladen trawler capsized off the coast of Greece, with an estimated 500 people on board; only 104 were rescued. How many missing migrants are there?