Jason Daley in Smithsonian Magazine:
In 218 B.C. the Carthaginian general Hannibal led an army of 30,000 soldiers, 15,000 horses and mules and 37 war elephants across the Alps into Italy, a bold move that led to one of the greatest victories of the Second Punic War with Rome. It placed Hannibal in the pantheon of legendary ancient generals like Alexander the Great and Julius Caesar.
The crossing is still studied by military tacticians today, but the details are a bit hazy. Historians have speculated for centuries about exactly what route the Carthaginian army took through the mountains, but there has been no solid proof. Now, microbial evidence from horse manure may point to Hannibal’s hair-raising route.
A study published in the journal Archaeometry shows that a “mass animal deposition” took place in the Col de Traversette, a 9,800-foot pass on the modern border between France and Italy around 200 B.C. Microbiologists from Queen’s University in Belfast sampled soil from a peaty area near the top of the pass, the type of place that an army might stop to water its horses. What they found was a disturbed layer of peat about 40 cm down that was not churned up by natural occurrences like a flock of sheep or frost, according to a press release.