In the 1840s and 1850s, the Mima Mounds drew the attention of travelers and explorers who recorded different ideas about their origin. In April 1840, Sir James Douglas of the Hudson’s Bay Company traveled through the region’s mounded prairies and wrote that the plain’s sinuous banks, knolls, slope toward the Chehaylis River, and Native oral tradition supported the view that it had once belonged to the region of waters. On May 20, 1841, Charles Wilkes, commander of the United States Exploring Expedition from 1838 to 1842, considered whether the mounds might be burial sites, opened three of them, and found only a pavement of round stones. On April 6, 1847, artist explorer Paul Kane camped in the Prairie de Bute and described innumerable round elevations touching each other like hemispheres across an extraordinary prairie he traveled through for twenty-two miles. In 1853–1854, James Graham Cooper, working in the area for the Pacific Railroad Surveys, suggested that eddies and whirlpools in a former estuary or sound system might have produced them. Upper Chehalis stories instead held that the mounds were left after a great flood subsided, linking their wave-like forms near Mima Prairie to the waters that once covered the land.