During the Second Punic War, Hannibal, in a brazen move, led a massive army over the Alps, surprising the Romans from the supposedly impenetrable northern border. The exact route Hannibal took is unknown, although some geographic information can be gleaned from historical accounts such as those of the Roman writer Polybius. Armed with this information, and the knowledge that tens of thousands of men, horses and elephants must have left some trace, geoscientists are hunting down possible locations using deduction and chemistry to test hypotheses.