The Nazca lines were made by clearing the dark oxidised stones of the desert pavement to expose the pale ground beneath, and because it almost never rains there they have survived. They fall into three kinds: long straight lines running for kilometres, geometric trapezoids and spirals, and the biomorphs, figures of a hummingbird, a monkey, a spider, a whale and others, which are the most famous and by far the least numerous.
Helaine Silverman and Donald Proulx's survey sets out the leading interpretation. Many of the lines converge on centres, and they are associated with water: they run towards the mountains from which the underground water comes, they are found near the spiral shafts that give access to the water table, and they were made in a place where water is the governing problem. The figures are drawn in a single continuous line that can be walked without crossing itself, which supports processional use, and the pottery deliberately broken along them points the same way.
Maria Reiche, who surveyed and protected them for decades from the 1940s, argued for astronomical alignments, and statistical testing has not supported that as a general explanation. The atlas records the ceremonial and water related reading as the position most sources now hold, and the astronomical one as tested and not sustained.