Nazca drawing were created by removing the top layer of stones and pebbles, dark from prolonged exposure to the elements, to reveal the lighter soil beneath. The "graffiti" was believed to have been committed in between the 100 BC and 700 CE. The fact they still remain demonstrates how little precipitation this area receives.
What what the hell do they mean?
The drawings were not discovered until airplanes began to fly over the area early this century. Since their discovery they have become one of the world's ancient unsolved mysteries. Of course, this being the case a number of interesting theories have been proposed.
In 1941, Dr. Paul Kosok, a professor of history, saw the setting sun touch the horizon just above the end of a straight line at the base of which he was standing. The thought occurred to him that perhaps the drawings were part of an elaborate network of astronomical calculations. Dr. Maria Reiche, a mathematician and astronomer, became an ardent proponent of this theory. She did most of her calculations as they were done in those archaic times, with a pencil. The advent of computers destroyed the theory when a series of tests proved that correspondence between celestial bodies and the drawings were sporadic at best.
Wonder what Dr. Maria Reiche is doing these days?
Erich Von Daniken developed his own theory about the drawings. This one involved a very interesting history of extraterrestrial visitors. He proposes that the Nazcas hosted visitors from outer space who were only able to stay briefly. The Nazcas enjoyed their company greatly and wanted them to return. In an attempt to lure them back, the Nazcas deepened the marks that the original landings had caused. When the ETs did not reappear, the dispirited Nazcas drew new ones, this time in the shape of sacrificial symbols, like birds, animals and insects.
This theory has not yet been fully accepted or recognized by the scientific community.
Source: Mysteries of the Past