July 4, 1805 was an eventful day for the Lewis & Clark Expedition. The men were busily engaged in portaging their boats, supplies and equipment around the Great Falls of the Missouri. To celebrate the Fourth, they were going to end the day with a dance and a feast. Captain Meriwether Lewis, hard at work on his ill-fated attempt to build an iron boat, was planning to issue the last of their liquor supply. Despite the busy day, Lewis took the time to record the following curiosity in his journal:
since our arrival at the falls we have 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. I was informed of it by the men several times before I paid any attention to it, thinking it was thunder most probably which they had mistaken at length walking in the plains the other day I heard this noise very distictly, it was perfectly calm clear and not a cloud to be seen, I halted and listened attentively about an hour during which time I heard two other discharges and tok the direction of the sound with my pocket compass.
At first Lewis thought the noise might be caused by water under pressure—an “Old Faithful” type phenomenon. “I have thout it probable that it might be caused by runing water in some of the caverns of those immence mountains, on the principal of the blowing caverns,” he wrote. “But in such case the sounds would be periodical & regular, which is not the case with this, being sometimes heard once only and at other times, six or seven discharges in quick succession. it is heard also at different seasons of the day and night. I am at a loss to account for this phenomenon.”
Weather was an obvious culprit, as the area around the Great Falls seemed to be a magnet for volatile storms. The next day Lewis recorded, “In the couse of last night had several showers of hail and rain attended with thunder and lightning. about day a heavy storm came on from the S W attended with hail rain and a continued roar of thunder and some lightning. the hail was as large as musket balls and covered the ground perfectly.” However, Lewis found that the strange noises were heard at odd intervals, including when the weather was perfectly calm. Clark, the expedition’s weatherman, also noted with puzzlement that on clear, cloudless days, “a rumbling like Cannon at a great distance is heard to the west if us.” Clark added, “the Cause we Can’t account.”
On July 11, Lewis recorded that he had heard the noise again:
this evening a little before the sun set I heared two other discharges of this unaccounable artillery of the Rocky Mountains proceeding from the same quarter that I had before heard it. I now recollected the Minnetares making mention of the nois which they had frequently heard in the Rocky Mountains like thunder; and which they said the mountains made; but I paid no attention to the information supposing it either false or the fantom of a supersticious immagination. I have also been informed by the engages that the Panis and Ricaras give the same account of the Black mountains which lye West of them. this phenomenon the philosophy of the engages readily accounts for; they state it to be the bursting of the rich mines of silver which these mountains contain.
Though he had initially pooh-poohed the accounts given by the Minnetare Indians, personal experience had convinced Lewis that the noise was real, even if his scientific mind could not immediately discern the cause. He wrote confidently, “I have no doubt but if I had leasure I could find from whence it issued.”
Lewis and Clark moved on and never did account for source of the noise. Later, other travelers corroborated the Corps of Discovery’s account of mysterious booming noises in the area of the Great Falls and the Black Hills of South Dakota. The Rocky Mountains are not unique in inspiring reports of unaccountable noises; it is not uncommon in mountain regions throughout the world. The rumbling, thunder-like noises heard in mountain regions are sometimes attributed to sudden avalanches, though it seems likely that Lewis would have readily identified this if it had been a plausible cause. Another possible explanation for sudden mountain booms is the natural creaking, groaning, and settling of the mountains themselves, as geographic forces converge and tons of rock presses in upon itself.
This is, however, as much a theory as Captain Lewis’s speculation. No definitive explanation for the cannon-like booms Lewis and Clark described has ever been found. The “artillery of the Rocky Mountains” remains a mystery to this day.