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.
A People With Few Traces
What's amazing, Sheets says, is that this technology should work at all in the Costa Rican rain forest. Satellite images have been used to find ancient human artifacts buried beneath the sands of the Sahara Desert of north Africa and ancient roads near Rome, where conditions are dry enough and vegetation is sparse enough to make it feasible. But no one was sure it would work in the rain forest.
The ancient people of that region had a simple lifestyle, with few monuments and few inhabitants, so their impact on the land was minimal. And all that plant growth should have wiped out any chance of detecting much from space. That challenge is precisely why NASA joined the effort in the 1980s.
"We thought if we could be successful here, we could do it anywhere," says Sheets, whose current research is sponsored primarily by the National Science Foundation.
The airborne imaging equipment demonstrated in the 1980s that it could be done, but day after day the aircraft and its crew of about 30 technicians and scientists waited on the runway for the cloudy skies to clear up. That turned out to be a very expensive effort, and it grated on the nerves of Sheets and other scientists involved in the project.
"The IKONOS satellite took away a huge amount of anxiety for us," Sheets says. It passes over the area once every six days, and when the weather is clear enough, "kaboom, we get gigabytes of imagery."