Jim McMahon/Mapman®

They are like constellations carved in mud. More than 1,000 years ago, Indigenous Americans left thousands of mysterious etchings on the ceiling of a cave in Alabama. The carvings stretch across more than 3 miles of tunnels. Many of their lines are so faint they are barely noticeable to the eye. But now experts are using an advanced 3-D scanning technique to identify them.

The etchings are glyphs—characters or symbols. One, more than 11 feet long, resembles a rattlesnake. Together, the glyphs make up the largest known collection of cave art yet found in North America. (Their exact location, called the 19th Unnamed Cave, is being kept secret to prevent damage from visitors.)

Researchers from a group called Ancient Art Archive recently published their findings about the glyphs. They believe that the etchings were made during an era when North Americans were beginning to settle into dwellings and raise crops. The artwork, researchers say, likely played a part in an Indigenous religion and represented spirits from the underworld.