Early last week, Greenpeace activists hiked into the Peruvian desert to the site of one of the Nazca lines, massive drawings that were made around 1,500 years ago in the rocks and sand. The activists unfurled a message in bright yellow letters.

“Time for Change!” it read. “The future is renewable.” After taking some photos with a drone, they packed up and left. The only problem? Their footsteps destroyed part of the ancient, irreplaceable artwork.

“The environment is very fragile,” says Patricio Valderrama Murillo, a Peruvian geologist who has studied the site. “That’s why it’s a restricted area, no one can walk or access there. It’s forbidden. Even the footprints you leave while you are walking in the desert are permanent.”

In a visit last week, archaeologists from Peru’s Culture Ministry documented the damage, noting overturned rocks and markings along the paths where the activists walked. The outline of a giant letter “C” from their message still shows faintly in the sand. In this video, new drone footage shows what went wrong.

The original drawings are very shallow; the land is covered with a thin layer of reddish rocks on top of sand, so even a small disturbance is immediately visible. “Greenpeace had about 20 people there, and they walked in a single line,” says Valderrama. “They actually created a new kind of line, and now you can see very clearly that there’s a new path there. The damage is going to be permanent.”

Since the area is so sprawling–the hummingbird figure that the activists stood next to is over 300 feet long–it isn’t fenced off. But signs clearly tell visitors to stay away. When Valderrama visited the Nazca lines for scientific research, he had special permission (and wore special shoe protectors that kept the ground undisturbed).

“It’s completely disrespectful to the Peruvian legacy,” he says. “We have significant monuments–Macchu Picchu and the Nazca lines–and Greenpeace attacked both. In 2008, they put their flags on Macchu Picchu. In this case, they caused permanent damage. I don’t know what they were thinking.”