Piranhas - the stuff of nightmares and legends. Theyre infamous for how fast they can devour prey, and are known to attack in groups of up to 100. Now, a team of scientists is heading to the Amazon to investigate a piranha mystery of immense proportions. A fossil of a giant jaw fragment has been identified as an ancient piranha, but with a terrifying twist: the tooth belonged to a fish seven times the size of existing piranha species.