Since the first ships put out to sea, sailors have told tales of sea monsters. Ancient Leviathan was thought to sleep beneath the waves, rising to swallow ships whole. Vikings feared the deadly Kraken, a creature that was half crab, half octopus. Giant sea snake attacks on ocean vessels were reported as recently as two hundred years ago. And though most such tales are now dismissed as legend, they are based in fact - long before humans walked the earth, real sea monsters ruled the deep.