Remarkably little is known about the European eel, Anguilla anguilla. So little, in fact, that scientists and philosophers have, for centuries, been obsessed with what has become known as the "eel question": Where do eels come from? What are they? Are they fish or some other kind of creature altogether? Even today, in an age of advanced science, no one has ever seen eels mating or giving birth, and we still don't understand what drives them, after living for decades in freshwater, to swim great distances back to the ocean at the end of their lives. They remain a mystery.