Satellite Imagery Locates Prehistoric Paths

May 29 -- For hundreds of years, the people who lived near the violent Arenal volcano in Costa Rica followed the same pathway, straight through the forest from their village to their cemetery, over and over again.

Beginning more than 2,500 years ago, the footprints of those early sojourners slowly carved a rut into the soil.

Now, all these years later, scientists are puzzled and amazed by an ancient pathway, long buried by volcanic ash and vegetation that has resurfaced again in satellite images from space.

Anthropologist Payson Sheets of the University of Colorado is on his way back to Costa Rica, along with a team of researchers, to see if the latest in satellite technology can help unravel the story of these ancient people.

"We've got a heck of a mystery here," says Sheets.

Finding Straight Lines

Part of the mystery involves how the pathways were discovered. They are almost impossible to detect from the ground, because years of growth and erosion have made them blend in with the surrounding area. But they show up clearly from space because the paths follow straight lines.

Nature abhors a straight line, and images from a NASA aircraft first made in 1984 intrigued Sheets and NASA archaeologist Tom Sever because a straight line is nearly always a clear indication of human activities. They traveled to the area and found that the line was indeed a pathway, but the images were few and far between and didn't show much of the area.

That changed last year with the launch of a commercial satellite, known as IKONOS. The IKONOS satellite took images of the footpaths in the visual and infrared portions of the light spectrum.

Vegetation growing over the pathway is different from adjacent growth. Thousands of footprints left a small trench, which filled up repeatedly during the eruptions of Arenal, creating a richer area for plant growth.

So the roots beneath the pathway are especially thick, producing more infrared radiation, which shows up as a faint, straight line in the images from IKONOS.