At first glance, it looks like an oil slick. But look closer and you'll see that it's a massive school of fish! This strange gathering of fish was spotted off the coast of Acapulco in Mexico. STOMPer Seng shares these pictures: "This was spotted off the coast of Acapulco in Mexico. "There is speculation that this strange behaviour might be caused by the change in ocean currents after the tsunami in Japan."