Ancient line drawings in the Andean desert, one of history's biggest mysteries, generated a firestorm of complaints against an environmental group that damaged the lines in a stunt this week.

The environmental group Greenpeace apologized on Wednesday for unfurling a sign calling for renewable energy next to a giant hummingbird design, one of the huge drawings called the Nasca lines. The message, spelled out in yellow plastic letters, was timed to get attention during a major climate conference in Lima. Greenpeace activists made an unauthorized intrusion into the World Heritage site to place the sign, resulting in protest from Peru.

Almost lost in the controversy were the Nasca lines themselves, which the United Nations Educational, Scientific and Cultural Organization (UNESCO) calls "one of the most impenetrable enigmas of archaeology." (Learn more about the lines in National Geographic magazine's "Spirits in the Sand.")

All together, more than a thousand straight lines, geometric shapes, and plant and animal figures dot the 174 square miles (450 square kilometers) of the Nasca Desert and foothills. Preserved by centuries of sun with little rain, the longest stretches more than 7.5 miles (12 kilometers), and the largest figure, a pelican, is some 935 feet (285 meters) long.

What were these lines? A puzzle since their 1920s rediscovery from the air, the images have long intrigued archaeologists (and UFO buffs), generating theories to explain their existence that range from ritual walkways to navigational pointers to marketplaces. (Related: "Andean Rock Art Pointed to Festival Sites in 300 B.C.")

View Images Greenpeace members are visible standing around their message, placed next to a hummingbird geoglyph. Photograph by Rodrigo Abd, Associated Press

Painted Desert

Careful removal of the dark varnished crust from the lighter-colored desert floor created the lines, called geoglyphs, made more than 1,500 years ago by ancient Andean people. Native Americans as far north as California used a similar method for creating desert drawings, most famously the Blythe "Intaglios," depicting large human figures.

Desert varnishes form very slowly, in some cases over tens of thousands of years. How these coatings, only about as thick as a sheet of paper, grow on desert rocks is poorly understood, according to Joseph McAuliffe of the Arizona-Sonora Desert Museum. Bacteria baking in the desert sun may actually concentrate darker minerals on exposed rock surfaces, he suggests, with the varnish essentially serving as a microbial sunscreen.

The Greenpeace activists drew Peru's ire as much for potential damage done to this delicate desert surface as for trespassing. The lines "are absolutely fragile," Peru's Deputy Culture Minister Luis Jaime Castillo told the Guardian: "You walk there and the footprint is going to last hundreds or thousands of years."

"The decision to engage in this activity shows a complete disregard for the culture of Peru and the importance of protecting sacred sites everywhere," said Greenpeace U.S. Executive Director Annie Leonard in a response to Peru's protest, which had included threats of detaining the activists. "There is no apology sufficient enough to make up for this serious lack of judgment."

This story was updated to include an additional apology from Greenpeace.