One of the most enduring archaeological mysteries of ancient Peru are the so-called Lines of Nasca or geoglyphs which consist of giant geometric forms (triangles, trapezoids, parallel lines) as well as biomorphs (birds, plants, and mammals) etched into the surface of the desert of southern Peru, especially in the drainage of the Río Grande de Nasca. The geoglyphs were constructed by clearing the surface of small stones darkened by desert varnish, exposing the lighter soil beneath. Some of the geoglyphs are over a kilometer in extent and are located in all parts of the drainage, although many are concentrated on the Pampa de San Jose, near the modern city of Nasca. The majority of the lines can be dated to the Nasca Culture (0 to 700 A.D.) on stylistic grounds (the biomorphs are identical in iconography to that painted on Nasca pottery--see Proulx 1968), and by the presence of broken fragments of Nasca pottery scattered on the surface of the geoglyphs. However, Clarkson (1990) has argued convincingly that some of the linear geoglyphs date to the later Middle Horizon (600-900 A.D.) and Late Intermediate Period (900-1476 A.D.), thus suggesting that the technology of constructing geoglyphs was cross cultural and formed a local tradition on the south coast of Peru.

First discovered in 1927 by Mejía Xesspe (1940) who thought they were ancient roads, the geoglyphs received only sporadic attention until the 1940's when American geographer and historian Paul Kosok visited the pampa and happened to observe the sun setting over the end of one of the lines on the day of the winter solstice (Kosok, 1965). He concluded that the geoglyphs served as an ancient calendar and that the lines marked the position of the sun at different times of the year or pointed to various stars or constellations. His disciple, Maria Reiche, spent the remaining 40 years of her life recording the geoglyphs and expanding on the astronomical theory (Reiche 1968, 1974 inter alia). Today this theory has been largely discounted as a result of the investigations of Gerald Hawkins, an astronomer with the Smithsonian Astrophysical Observatory. Using modern computers, Hawkins found only random correlations between the position of the geoglyphs and the astronomical features (Hawkins, 1969). Anthony Aveni (1990), an archaeoastronomer at Colgate University, has also argued compellingly against the astronomical theory.

Over the years many speculative explanations have been proposed for the function of the geoglyphs. Among these are several which are completely unscientific and deserve little attention: Von Daniken's (1968) argument that the lines served as landing strips for visits by ancient astronauts; Rosell Castro's idea (1977) that they were remnants of cultivated fields; and Henri Stierlin's suggestion (1983) that the cleared areas served as giant textile workshops. A more credible theory, based on ethnographic analogy, proposes that the lines were used as ritual pathways leading to sacred locations (Morrison 1978; Silverman 1990; Aveni 1990, 1996). This model includes arguments pointing out the similarity between the Inca "ceque lines" (a series of invisible lines radiating out from the center of the capital city of Cuzco connecting shrines as well as delineating social or kin-related space) to similar radiating Nasca Lines or "star clusters" present on the Pampa de San José (Urton 1990; Aveni 1990).

Finally, there are theories that involve the correlation of the geoglyphs with water in some manner. Mejia (1940) was one of the first to imply a similarity between the linearity of the geoglyphs and filtration galleries or puquios present in the Nasca Valley. Rossel Castro (1977:209-216) argued that just as the aquifers formed an irrigation network underground, the geometrical figures delineated a pattern of ancient fields and irrigation systems on the surface. However, modern studies using aerial photography have not supported this hypothesis. In 1986, Aveni published a study indicating that the orientations of the triangles and trapezoids are statistically correlated with the flow of surface water, and that taken as a whole, the lines are most likely associated with "a ritual scheme involving water, irrigation and planting" (Aveni 1986:39). Reinhard (1996:56) using ethnographic and ethnohistorical data, has argued that a strong relationship exists in Andean tradition for a connection between water, fertility and mountain worship. He suggested that "some lines, especially the large triangles and rectangles, may well have served as symbolic connectors with water sources (rivers, mountains, the ocean) and were sacred areas where fertility rites were carried out ."

In 1996, Johnson (1999; 1998) proposed a new explanation of the function of the geoglyphs based on his fieldwork and observations in the Nasca drainage. This hypothesis argues that some of the geoglyphs mark the path of aquifers which carry water through geological faults. This is not an unreasonable explanation given the geologic setting. This area of Peru is located in one of the most active seismic zones of the world. Faults are common. These faults are an integrated and interconnected network that can collect water in one part of the region and conduct it across the valleys to locations where it can be reached by digging puquios or wells, or to locations where the water table is high enough for springs or seepage to be present on the surface. Due to insufficient surface water in the river system, the ancient inhabitants of the drainage settled in locations adjacent to geological faults which provided water from the aquifers. Thus, there is a strong correlation between archaeological sites, geological faults, aquifers, fresh water and the geoglyphs which mark their location.

This hypothesis differs significantly from other "water-related" explanations in that all earlier models were based on the location and flow of surface water while Johnson's hypothesis recognizes the importance of subsurface water flow and the role of the structural geology and hydrology in understanding the mechanism of its transmission. The Pre-Columbian inhabitants of the drainage, through familiarity and a basic understanding of their environment, were able to use geoglyphs to mark the location and flow of the aquifers.

A team of scientists and experts was assembled to test this new hypothesis. The team consists of David Johnson, Donald Proulx, an archaeologist specializing in Nasca culture and Stephen Mabee, a geologist with expertise in hydrogeology. The purpose of assembling this team was to apply standard hydrogeological, geological and geophysical techniques in concert with archaeological methods and gather evidence that would either support or refute the hypothesis proposed by Johnson (1999; 1998). Proulx has conducted an archaeological survey in the lower Nasca and Río Grande valleys while Johnson and Mabee have examined the geology of the subterranean hydrologic systems using a combination of geologic mapping, shallow subsurface geophysical techniques, surveying and water quality analysis.

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