In July 1808, 100 kilometers east of the Montana Rockies, Lewis and Clark wrote, “We have repeatedly heard a strange noise coming from the mountains. … It is heard at different periods of the day and night … and consists of one stroke only, or of five or six discharges in quick succession. It is loud and resembles precisely the sound of a six-pound piece of ordnance.”

They were leading the first overland expedition of the United States territory, so it wasn’t a cannon. The sounds have never been explained.