Now, digital analysis of rock surfaces reveals how other ghosts of the deep past—this time from almost 2,000 years ago in North America—have been coaxed into the light. Writing in the journal Antiquity, Professor Jan Simek of the University of Tennessee and colleagues have published images of giant glyphs carved into the mud surface of the low ceiling of a cave in Alabama.

The motifs, which depict human forms and animals, are some of the largest known cave images found in North America and may represent spirits of the underworld. In the first image above, a drawing of a diamondback rattlesnake, an animal sacred to indigenous people in the south-eastern U.S., stretches almost 3 meters long. The image below shows a human figure just over 1.8m in length.

In terms of dating the findings, ancient people rejuvenated a light in the cave (a flaming torch of American bamboo) by stubbing it against the cave's wall. This left a residue that the researchers were able to date with radiocarbon to 133–433 AD. This was also in accord with the age of pottery fragments ancient artists left in the cave.

The problem is seeing the paintings. The cave ceiling is only 60cm high, which makes stepping back to view the large images impossible. They were revealed only through a technique called photogrammetry, in which thousands of overlapping photographs of an object or place are taken from different angles and digitally combined in 3D. Photogrammetry is a cheap technique increasingly used in archaeology to record artifacts, buildings, landscapes and caves. It allowed Professor Simek's team to "lower" the cave floor up to 4 meters, enough for the complete motifs to come into view for the first time.