The cosmos of the ancient Earthworks Cultures of the Eastern Woodlands was a layer-cake type of division of realms. The three most important divisions were the Sky World above, the Earth Disc where all humans, plants, animals, etc. live (aka “Earth”), and the Underworld.
The Underworld was a watery place inhabited by powerful entities depicted by both ancient and historic Native Americans as Great Horned Serpents or Underwater Panthers.
These beings were ruled by a singular chief of the Underworld Powers, known by a number of names. As explained by the great ethnographer and archaeologist George Lankford:
"The most widely recognized form of the underwater powers, however, is the Horned Water Serpent. It is frequently referred to as a single creature, but more careful reading through the collections indicates that the Underwater Panther/Horned Water Serpent is really understood to be a race of people, representatives of which may be encountered in any large body of water, whether seas, lakes, or rivers. Even so, there is still frequent reference to a “master” of the underwater powers." (1, p. 109)
Since the Native American cosmos does not draw artificial barriers between the three realms and considered them interconnected, the Underworld powers could—and did—access the Earth Disc. The Great Serpents emerged from streams, lakes, springs, and caves. They were usually dangerous to man, causing drowning, floods, and other calamities, but they could also offer powerful magical abilities. This tradition is still alive today, as many Native medicine societies maintain some type of connection to the Master of the Great Serpents.
There is an account of an actual sighting of the Great Serpent/Underwater Panther by the Sioux in the Missouri River, recorded in the late 1800s:
“Long ago the people saw a strange thing in the Missouri River. At night there was some red object, shining like fire, making the water roar as it passed up stream. Should anyone see the monster by daylight he became crazy soon after, writhing as if with pain, and dying. One man who said that he saw the monster described it thus: It has red hair all over, and one eye. A horn is in the middle of its forehead, and its body resembles that of a buffalo. Its backbone is like a cross-cut saw, it is flat and notched like a saw or cog-wheel. When one sees it he gets bewildered, and his eyes close at once. He is crazy for a day, and then he dies. The Tetons think that this monster is still in the river, and they call it ‘Mi-ni-wa-tu’, or sea monster. They think that it causes the ice on the river to break up in the spring of the year.” (1, pp. 115-116)
1. George E. Lankford, “The Great Serpent in Eastern North America”, Ancient Objects and Sacred Realms: Interpretations of Mississippian Iconography, ed. F. Kent Reilly and James F. Garber, University of Texas Press, Austin, 2007, pp. 107-135.