The Unaccountable Artillery:

Montana's Two-Century Mystery of the Phantom Cannon Fire

A SOUND WITHOUT A SOURCE

In the early days of July 1805, the Corps of Discovery was pinned down at the Great Falls of the Missouri River, hauling canoes and gear across eighteen miles of broken prairie in one of the most physically punishing episodes of the entire expedition. The men were sunburned, thorn-pierced, and exhausted, their moccasins shredded by prickly pear. It was in the midst of this ordeal, amid the roar of the falls themselves, that Meriwether Lewis first paused to record something stranger than fatigue: a sound like distant cannon fire, rolling in from an empty horizon. No army was near. No storm was visible. And no one, then or since, has offered an explanation that has fully satisfied historians or scientists. Two centuries later, the puzzle Lewis called the "unaccountable artillery of the Rocky Mountains" remains one of the more curious footnotes to the expedition's Montana passage, discussed by geologists, acousticians, and folklorists alike, and never definitively solved.

THE CAPTAINS' OWN WORDS

The clearest account comes directly from Lewis's journal, written at the White Bear Islands camp above the falls. He noted that since the party's arrival there, they had "repeatedly witnessed a nois which proceeds from a direction a little to the N. of West as loud and resembling precisely the discharge of a piece of ordinance of 6 pounds at the distance of three miles." He admitted that his men had mentioned the noise to him several times before he paid it any attention, assuming they had mistaken thunder for something more exotic. Then, walking the plains himself, he heard it directly, under conditions that ruled out the obvious explanation: the sky was clear, the air was calm, and there was not a cloud in sight. Lewis confessed himself at a loss, though he retained the confidence of an Enlightenment-minded naturalist, writing that given sufficient time he had no doubt he could trace the sound to its source. He never got the chance. On July 11, according to the same account, he recorded two further discharges of what he now termed, half in jest, this "unaccountable artillery" of the mountains.

Clark's version of events, recorded independently, corroborates Lewis almost word for word. He described a rumbling like cannon fire at a great distance, coming from the west, adding simply that its cause could not be explained. The convergence of two trained, disciplined observers, each accustomed to distinguishing gunfire, thunder, and the roar of moving water after more than a year on the trail, lends the account unusual weight. These were not men prone to superstition or exaggeration in their field notes; they were surveyors and soldiers whose daily entries dealt in distances, bearings, and provisions. That both captains devoted attention to an unexplained noise, and that both used almost identical language to describe it, suggests they took the phenomenon seriously rather than dismissing it as camp talk.

A SOUND ALREADY KNOWN TO OTHERS

What makes the Great Falls episode more than an isolated curiosity is that the expedition had heard rumors of it before ever reaching Montana. Wintering among the Hidatsa and Mandan villages the previous year, Clark recalled being told that the Rocky Mountains were known to make a great noise, though his informants could not explain the cause or point to any physical feature responsible for it. Clark went further, speculating that the Sun River, which the expedition initially called the Medicine River, may have taken its name from this very phenomenon, since anything inexplicable was, in the language of the Missouri River tribes, described as "medicine." He noted as well that the Arikara people, farther downriver, reported a similar noise originating from the Black Hills of present-day South Dakota, a mountain range some three hundred miles to the southeast of the Great Falls. Lewis, writing separately, recorded that Pawnee and Arikara informants had likewise spoken of the phenomenon, attributing it not to weather or geology but to underground deposits of silver ore, whose extraction by unseen forces was thought to produce the sound.

This detail matters because it demonstrates that the Great Falls booms were not a singular event tied to one place or one set of ears. Indigenous peoples across a wide swath of the northern plains had independently associated the sound with the mountains long before Lewis and Clark arrived, and continued to report it in locations far removed from the Missouri River portage. The recurrence of similar accounts among unrelated groups, describing a phenomenon none of them could see or touch, argues against simple explanations involving imagination or coincidence, even as it complicates the search for a single physical cause.

A PHENOMENON WITH DEEP AND WIDE ROOTS

The mystery did not end with the expedition's departure from Montana. In August 1872, a survey party working for the Northern Pacific Railroad, camped at the mouth of the Sun River under the direction of engineer Thomas P. Roberts, reported hearing sounds strikingly similar to those Lewis and Clark had described nearly seven decades earlier. Roberts confided to his own journal that there was something strange in the coincidence. Trappers and settlers in the intervening years apparently reported comparable experiences, though their accounts survive mainly in secondhand references rather than in dated field journals.

Nor is the phenomenon unique to Montana. Similar booming sounds, unaccompanied by visible cause, have been documented for centuries around the world under a bewildering array of local names. Researchers cataloguing the reports found equivalents from Ontario to Bangladesh to West Africa, each culture having independently coined a term for noises that resembled artillery fire, thunder, or subterranean groaning, with no storm, army, or blasting operation to account for them. In New England, similar sounds were reported around Moodus, Connecticut, and West River Mountain, New Hampshire, well before the Lewis and Clark expedition ever set out. In the late 1970s, hundreds of unexplained "mystery booms" were reported along the eastern seaboard of North America over several months, drawing enough public alarm that a United States senator requested a formal investigation by the Naval Research Laboratory, which attributed roughly two-thirds of the recorded events to supersonic aircraft and left the remainder unresolved.

EXPLANATIONS OFFERED, AND DISCARDED

The search for a scientific explanation began almost as soon as the expedition returned. Nicholas Biddle, editing the captains' journals for publication, initially proposed to Clark that the sounds might resemble avalanches in the Alps, snow and ice collapsing down steep mountain faces with a roar comparable to distant cannon fire. Clark seems to have entertained the idea in correspondence, but Biddle ultimately left it out of the published History of the Expedition in 1814, apparently unconvinced by his own suggestion. The avalanche theory, while plausible for winter conditions in genuinely steep terrain, does not easily explain a summer phenomenon heard on the relatively gentle plains bordering the Great Falls.

Twentieth-century researchers turned to seismology. In the 1930s, following an earthquake near Helena, geologists proposed that certain sounds accompanying small tremors, produced by so-called P waves reaching the surface, might account for phenomena of this kind. The trouble, as later analysts pointed out, is that the country around the Great Falls sits in an area of relatively low seismic activity, making frequent earthquake-generated booms an awkward fit for a location where the noise was reportedly heard repeatedly within the space of a single week. A more geologically active area, such as the Three Forks region roughly two hundred miles upstream, would seem a more probable candidate for tremor-related sound, yet it is not the location where Lewis and Clark documented their experience.

A different line of inquiry, advanced by the physicist Jearl Walker in the 1970s, proposed an atmospheric explanation grounded in acoustics rather than geology. Ordinary thunder, Walker noted, fades to inaudibility within about fifteen miles under normal atmospheric conditions. But temperature inversions, in which a layer of cold air is driven beneath warmer air, can bend sound waves traveling upward back down toward the earth at considerable distances, in much the same way that a whispering gallery carries a faint voice across an open room. Under the right combination of weather and topography, this effect can make distant thunder, or the concussion of an unrelated event entirely, audible many miles from its source, in places where the storm itself was never seen. Supporting evidence for this kind of acoustic anomaly comes from documented instances during the Civil War and the First World War, when battle sounds were clearly heard more than a hundred miles from the fighting while observers standing much closer heard nothing at all.

A more recent scientific contribution came from the astrophysicist Thomas Gold and his colleague Stephen Soter, who examined the global history of these so-called brontides and proposed that many could be explained by the sudden release of pressurized natural gas from underground reservoirs, a process that could occur independently of conventional earthquake activity and might account for booms heard in regions with little recorded seismicity. Their analysis, published in the journal Science, did not claim to resolve every reported case, and the Great Falls phenomenon specifically was not part of their study, but it added a plausible geological mechanism to the list of candidate explanations circulating among researchers.

CONCLUSION

No single theory accounts for every detail of what Lewis and Clark heard above the Great Falls of the Missouri in the summer of 1805. Avalanche, seismic tremor, atmospheric refraction of distant thunder, and subterranean gas release have each been proposed, and each fits some portion of the historical and geographical record while leaving other parts unexplained. What is not in doubt is the consistency of the testimony. Two disciplined observers recorded the same sound independently, under clear skies, on more than one occasion, in a location where Indigenous peoples across a wide region had already reported comparable experiences long before the expedition's arrival, and where later travelers claimed to hear it again decades afterward. The phenomenon belongs to a broader category of unexplained natural sound documented on nearly every continent, one that modern science has narrowed without fully closing. For a Montana curious about its own mysteries, the artillery of the Rocky Mountains offers a rare case in which the historical record is unusually solid, even as the physical explanation remains, in Lewis's own words, a matter he was never given the leisure to resolve.

Works Cited

Gold, Thomas, and Stephen Soter. "Brontides: Natural Explosive Noises." Science, vol. 204, no. 4391, 1979, pp. 371-75. www.science.org/doi/10.1126/science.204.4391.371. Accessed 10 Aug. 2026.

Clark, William. Journal entry, 20 June 1805. The Journals of the Lewis and Clark Expedition, edited by Gary E. Moulton, University of Nebraska-Lincoln, lewisandclarkjournals.unl.edu/item/lc.jrn.1805-06-20. Accessed 10 Aug. 2026.

"Artillery Sounds Heard By Lewis and Clark." National Park Service, U.S. Department of the Interior, 7 July 2023, www.nps.gov/articles/000/artillery-sounds-heard-by-lewis-and-clark.htm. Accessed 10 Aug. 2026.

"Lewis and Clark in Montana: The Great Falls of the Missouri." Montana Bureau of Mines and Geology, Montana Technological University, www.mbmg.mtech.edu/Pubs/Lewis-Clark/lewis-clark-greatfalls.asp. Accessed 10 Aug. 2026.

Mussulman, Joseph A. "Soundscapes: The Sonic Dimensions of the Lewis and Clark Expedition." We Proceeded On, vol. 21, no. 4, Nov. 1995, pp. 13-19. Lewis and Clark Trail Heritage Foundation, lewisandclark.org/wpo/pdf/vol21no4.pdf. Accessed 10 Aug. 2026.

Mussulman, Joseph A. "Unaccountable 'Artillery' of the Rockies." Discover Lewis & Clark, Lewis and Clark Trail Alliance, lewis-clark.org/sciences/geography/unaccountable-artillery-of-the-rockies/. Accessed 10 Aug. 2026.