Specifically, latrines, reports Esther Inglis-Arkell for io9.com. Related Content When Seals Molt, They Leave Behind Mercury.
Lewis and Clark and their team stopped at more than 600 sites, according to their journals.
Though many were home only for a day, each would have had pits dug to hold their waste.
It turns out that the expedition was well-equipped with the best medicines of the day, which gave each of those latrines a unique mercury-laden signature.
The pills were so strong that people called them "thunderclappers" or "thunderbolts," reports Maurice Possley for the Chicago Tribune.
Experts used this information to pinpoint the location of the campsite just south of modern-day Missoula, Montana.
The others include more permanent forts and Pompeys Pillar on the Yellowstone River near Billings, Montana where Clark carved his initials. »