Most people know that the blue whale is the biggest animal that ever lived. With a length of 30 metres and a weight of 170 tons, it is bigger than any dinosaur and almost as big as a passenger plane. The immense size of the blue and many other whale species mean that they have few natural predators. But it was not always so. There was a time, before the existence of whales, when gigantic sea monsters ruled the oceans - and some were even as big as the blue whale itself.