What is known and what is not
Scholars agree on who made the Nazca Lines and roughly when: people of the Nazca culture and some of their predecessors, over many centuries around the turn of the Common Era. They also agree on how: by removing dark surface stones to show the paler ground. Beyond that there is no consensus. The question of purpose has produced several competing ideas, and it is quite possible that more than one is partly right, because the geoglyphs were made over a long time and include very different kinds of design.
The astronomical calendar idea
The idea that the lines served as a giant calendar goes back to the early researchers. In 1941 the American historian Paul Kosok noticed that one line seemed to point to the place where the sun set on the winter solstice, and he is often quoted as calling the plain the largest astronomy book in the world. Maria Reiche, who worked there for decades, developed the idea and argued that many lines and figures marked the positions of the sun, moon and stars.
Later tests were less encouraging. In the 1960s the astronomer Gerald Hawkins compared line directions with sun and star positions using early computer methods and did not find that astronomical targets explained them better than chance would. Subsequent work by Anthony Aveni and others reached a similar view: a few lines may align with the sun at solstices, but most do not seem to have a consistent astronomical target. Many researchers therefore treat astronomy as, at most, a partial explanation. The same caution applies when claims are made about alignments at other ancient sites, such as the long-running debates over pyramid alignment in Egypt.
Ritual pathways
A second idea, developed by Aveni and others, is that the lines were paths for walking in ceremonies. Many lines lead to or from the line centres, where several converge. Some of the zigzags, spirals and single-line figures suit being walked, and a person following them would be moving through a formal sequence. Similar sacred pathways are known elsewhere in the Andes, including the ceque lines radiating from Cuzco in Inca times, though the Inca were many centuries later, so this is an analogy rather than a direct link.
This view fits with the ceremonial centre of Cahuachi and with the possibility of pilgrimage. It is attractive because it matches the physical form of the markings. It is difficult to prove, since walking leaves little trace on stony ground.
Water and fertility
A third family of ideas ties the geoglyphs to water. The Nazca lived in one of the driest regions on Earth, dependent on river water that came from the highlands and on groundwater tapped by underground channels. Researchers such as Johan Reinhard have proposed that lines and shapes were part of rituals for water, fertility and the mountain spirits who were believed to control it. Evidence comes from the way some lines point toward the mountains or toward water sources and from the prominence of water-related imagery on pottery. Excavations near Palpa have found offerings that suggest ritual activity associated with the geoglyphs.
The Nazca water ritual theory is therefore a major contender, particularly for the lines and trapezoids. It remains an interpretation based on pattern and analogy, not a demonstration.
Other proposals
Other suggestions include that lines marked territorial boundaries or routes between settlements, that some trapezoids were spaces for gatherings, and that certain figures were linked to clan or group identity. Some researchers combine ideas, proposing that lines were walked in rituals that were also connected with water and with seasonal cycles. These are not mutually exclusive.
Why the alien explanation is rejected
Popular books since the late 1960s have claimed that the Nazca Lines were landing strips or signals for visitors from space. Archaeologists reject this for several reasons. The lines are shallow marks on the ground that would not support a landing; the surface is a loose layer of stones. The makers are known: pottery fragments and wooden stakes link the work to the Nazca and earlier peoples, and radiocarbon dating of the stakes places the work in the human past. Experiments show that people with simple tools could build the designs. Finally, the claim relies on the idea that ancient South Americans could not have created such work, which has no evidential basis and undervalues the engineering and artistic ability of Andean societies. The wider topic of ancient astronaut claims is a modern idea rather than a hypothesis supported by the evidence from the site.
It is also sometimes said that the figures can only be seen from the air. In fact many can be viewed from nearby hills, and the designs may well have been experienced by walking along them, rather than seen from above.
Where research goes next
Progress is likely to come from excavation and dating, from detailed survey of newly found designs, and from comparing the geoglyphs with pottery imagery and with settlement patterns. Responsible interpretation also requires protecting the site so that evidence is not destroyed. At present the honest summary is that the purpose is debated and not fully understood.