But two archeologists who have pored over the patterns for the past five years say they may have unraveled the riddle.

And fittingly, the puzzle contains a paradox: According to one of the archeologists, Johny Isla, the famous Nasca lines (often spelled Nazca) and less well-known Palpa lines nearby, were all about water in one of the world's driest deserts.

"The main meaning is oriented to water. Water is life, fertility," Isla told Reuters, overlooking a double spiral etched into the gray-brown Palpa plain. He said it was often dubbed the sun dial but was in fact a sign linked with water.

Scientists and aficionados over the years have come up with plenty of theories to explain away one of Peru's top tourist attractions. One of the wackiest had the lines as landing strips for alien astronauts and their spaceships, while other experts thought they were sacred paths, the outlines of an agrarian calendar or linked to a fertility and mountain cult.

Isla, who has been studying and excavating around Palpa with his German colleague Markus Reindel since 1997, noted that the giant "geoglyphs" -- including birds, figures, trapezoids and spirals -- lie on a plain cut by three rivers, something that would have made it a very fertile "privileged site."

He said it appeared that people living on the Palpa plains chose where to settle based on where their water sources were, since the rivers would not have been full all year round.

Some trapezoids seem to point east or northeast, toward the source of the rivers, and two in particular point to a confluence of water that could be another clue, he said.

Furthermore, his team's excavations of mounds covered over at the end of some of the trapezoids, and of tombs, turned up offerings such as fragments of orange spondylus shells that can be found in Ecuador during the El Nino weather phenomenon and which have been considered symbols of water and fertility in the Andes for thousands of years.

Other finds included crab claws -- further evidence of the existence of a water cult.

"We know (water) was the principal function ... The theory of extraterrestrials is something we didn't even take into consideration. It makes no sense," Isla said.

WHODUNNIT?

The studies by Isla and Reindel, which have been funded by the Swiss-Lichtenstein Foundation for Archeological Investigation Abroad, have also filled in other important information for those trying to read between the lines -- such as the key question of who made them.

Until now, scientists had assumed the lines were made under the Nasca civilization, which ran from 200 B.C. to 650 A.D.

"Who did this is something that has never been known. It has always been said that they (the Palpa lines) belonged to the Nasca (period) because a lot of figures are reproduced (in the Nasca lines)," Isla said.

But his study's excavations, dated using a system of relative chronology that he said had a margin of error of 100 years, showed the area was inhabited much earlier. The excavations were the first conducted of Palpa or Nasca lines.

"The first geoglyphs were in this region (Palpa), at least the ones we know about today," Isla said. "They started here and spread to Nasca," he added, though he noted that not all the Nasca lines were younger than the Palpa ones.

Accordingly, the first Palpa lines dated from around 200 B.C., the end of the Paracas culture -- a civilization famous for its textiles, which flourished from 800 B.C. to 200 B.C. Although the earliest Palpa lines were formed in a slightly different way than the Nasca lines, and were thicker, Isla said some of the lines that crisscross the desert are connected over 9 miles of plain.

That helped debunk a theory that the Nasca people lived independently in valleys in simple societies.

"We totally disagree ... There's too much coincidence," Isla said, adding he believed the societies, whose famous desert lines often reproduced symbols that decorated their fine ceramics, were complex and highly hierarchical.

The study also identified and excavated what Isla said were key religious and administrative sites from the early- and mid-Nasca period -- called Los Molinos and La Muna, respectively -- although tomb-robbers had long ago plundered them.

RITUALS AND IRRADIATION

The lines -- which have survived for centuries largely intact -- were also used for rituals and were remodeled, according to the findings of the excavations.

Isla said broken ceramics and musical instruments found near the lines supported the theory they were used in rituals.

"They (the lines) weren't just made once and then left, but were used throughout time. Some were erased or remodeled or lines were superimposed ... These were social spaces, used for ceremonies and working," Isla said.

The so-called sun dial, for example, is crossed by a straight line and one symbol of a bird appears to have been partially obliterated by a giant trapezoid.

Some destruction is more recent -- the result of peasants trailing their animals across them or more willful destruction. Next to the sun dial, for example, Isla showed where locals had cleared an area, destroying part of a geoglyph, to make a soccer field, until they were stopped.

The Nasca lines are a U.N. world heritage site -- although poor policing has failed to crack down on vandals that have defaced them -- but the Palpa lines have no protection, something Isla said his team was lobbying to change.

Despite the new answers, Isla said more questions remained and said scientists were planning to use a revolutionary irradiation technique to pinpoint the lines' ages by measuring when the stones they contained were last exposed to daylight.

Because the lines were made by pushing surface shale on top of other stones, irradiating those that were hidden could prove a more accurate dating method, he said.

As for another of the key questions -- why bother to build elaborate constructions that are best appreciated from the air, when air travel was unknown to the ancient peoples -- Isla noted that the principal Nasca god was often shown in flight or related to divinities like the condor.

"I imagine they didn't need to see (the lines) -- they were offerings to the god. It was all for him," he said.

His team has paused its studies to write up the findings so far, and Isla said he was delighted to have discovered far more than he had imagined possible.

But he added: "We definitely don't think everything has been solved yet. If so, all the fun would be over."