The epic venture undertaken by Meriwether Lewis and William Clark to find a passage from the Missouri River to the Pacific left Camp Dubois in Missouri on May 14, 1804, and reached the end 28 months later. To trace that historical journey, experts rely not only on the expedition journals and maps, but also signs of the more than 30-person group’s passage that persisted over the centuries. Specifically, latrines, reports Esther Inglis-Arkell for io9.com.

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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. But how do you tell one pit latrine from another? 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.

For Mental Floss, Kirsten Fawcett writes:

Today’s doctors would shudder at the thought of patients ingesting what’s essentially mercury-poisoning-in-a-pill. But during the 18th century, calomel was a go-to drug for many conditions, including constipation. And sure enough, Lewis and Clark’s journals mention their men taking a popular remedy called Dr. Rush’s Bilious Pill—a fast-acting purgative that contained a whopping 10 grains of calomel per serving.

The pills were so strong that people called them "thunderclappers" or "thunderbolts," reports Maurice Possley for the Chicago Tribune. The mercury would have killed bacteria, but don’t try this remedy today because it also poisons humans. The element also doesn’t decompose, hence its presence in the latrine pits to this day.

Experts used this information to pinpoint the location of the campsite just south of modern-day Missoula, Montana. There, on the banks of Lolo creek they found mercury in an old latrine, located the proper distance (as suggested in a military guidebook Lewis and Clark used) from an old puddle of melted lead and a fire-cracked rock — the campfire where someone likely repaired a weapon. The expedition leaders called the site Travelers’ Rest. It is one of the only campsites to be identified. The others include more permanent forts and Pompeys Pillar on the Yellowstone River near Billings, Montana where Clark carved his initials. The Missola-adjacent site is now the location of Travelers’ Rest State Park.

Possley reported in his 2005 story that a pamphlet from the state park mentions the latrine: "The two men who hovered over this spot in 1806 probably did not feel so fortunate, but for the researchers and lovers of history, their misfortune is our triumph!"