Archaeologists make a grisly find: Four hundred skeletons buried in a mass grave. The bodies have lain undisturbed for 500 years, since the time of the Spanish conquest. But this is no ordinary gravesite. The remains suggest these people met a gruesome end at the hands of the Aztecs, who ruled Mesoamerica in the 14th through 16th centuries. But who were the victims and why were they killed?