It looks like an oil spill, but it's not. It's anchovies. That's right, a school, a group, an aggregation of anchovies, millions of them, stretching for more than a mile, off the coast of San Diego, California.

Oceanographic student Julia Fiedler took this amazing video and actually swam into the massive school, all the while keeping a sharp eye for leopard sharks feeding on the enormous - and free - buffet.

Scientists say the anchovy aggregation is the largest seen in the San Diego area in decades and have no idea what causes the gigantic group to form.