Sometime in the 12th century, on a forested hillside in what is now east Tennessee, a small band of Indians slithered through a hole and began using the mud-coated walls of a cave as a sketchbook.
Sometime in 1979, a Park Service ranger named Walter Merrill, exploring beneath the same hill, sloshed though an underground stream and discovered what the Indians had left: a richly decorated chamber unlike any known in the world.
''This is the only cave we know where (pre-Columbian man) drew on the mud,'' said Dr. Jon Muller, an anthropologist from Southern Illinois University at Carbondale, at a National Geographic announcement of the find. ''It's unusual because of the flexibility of the media. It really is like a sketchbook. And the quantity of these drawings is amazing.''
Other caves in the eastern United States, used by Indians as mines or living quarters, have yielded prehistoric artifacts. A few have drawings laboriously etched in rock.
But Mud Glyph cave, as scientists have dubbed the site, was apparently used only for religious rituals, or as an art gallery, between the 12th and 16th centuries. Its closest parallel may be caves in the south of France which contain Ice Age art.
''When we first heard about it, we were all quite skeptical,'' recalls Patty Jo Watson, a professor of anthropology at Washington University in St. Louis, ''but the minute we saw it, we knew it was real.''
The cave's inner chambers are covered with drawings, many overlapping or erasing others to form a difficult-to-decipher palimpsest of symbols. The act of drawing, say scientists, was probably just as important as the drawings themselves.
The most common drawing is simply a series of squiggles. ''It's very difficult to determine what the Indian had in mind when he drew this,'' says Bill Deane, a photographer who has recorded the cave.
The symbols then range upwards in sophistication, through animals (turtles, snakes, possums) to humans (simple stick figures, detailed warriers, and a man nicknamed ''The Jogger'' because he appears to be wearing running shorts.)
The drawings were probably made and viewed by a relatively small number of Indians, from a group known to specialists as ''the Dallas culture,'' and were likely intended to invoke some sort of magic aimed at gaining more power in society.
The scientists who explored the site, funded by the National Geographic, are greatly disturbed by one symbol: a deep etching that says ''KEN.''
Professional pot hunters, one apparently named Ken, sawed through a protective entrance three weeks ago and left ditches and graffiti in the cave's inner chambers.
''A simple swipe of the hand would destroy these drawings,'' says Charles Faulkner, a University of Tennessee anthropologist and leader of the scientistic study team.