Hundreds of mysterious Stonehenge-like earthworks have been discovered in the Amazon rainforest, according to researchers who have investigated more than 8,000 square miles in Brazil’s northwestern state of Acre.

The large geometric, ditched enclosures had long remained hidden by trees. But modern deforestation, in combination with Google Earth technology, have now revealed the presence of the more than 450 geoglyphs.

The carved ditches measure up to 36 feet wide, 13 feet deep and are 300 to 1,000 feet in diameter. The features “rival the most impressive examples of pre-Columbian monumental architecture anywhere in the Americas,” Jennifer Watling, a post-doctoral researcher at the Museum of Archaeology and Ethnography at the University of Sao Paolo, and colleagues wrote in the journal Proceedings of the National Academy of Sciences.

Excavations of the geoglyphs, which show a highly formalized architecture made of geometrical circles and squares, suggest they were used occasionally as public gathering sites to carry out ritual or ceremonial activities some 2,000 years ago.

The outer ditch and inner wall enclosure of the geoglyphs identify them as henge sites similar to Stonehenge, which is around 2,500 years older.

“We are still a long way from answering what the enclosures meant to the geoglyph builders themselves,” Watling told Seeker. However Watling added, “We are quite sure that they were not sites of permanent villages, nor defensive structures, due to both the forms of the earthworks and the small quantity of cultural remains that we discover when excavating them.”

Indeed, the almost complete lack of cultural material within the geoglyph area, suggests the enclosures were kept ritually “clean.”

The discovery of the huge earthworks has significant implications, not only for their puzzling meaning, but also because it overturns the notion of Amazonian rainforests as a pristine wilderness.

“We immediately wanted to know whether the region was already forested when the geoglyphs were built, and to what extent people impacted the landscape to build these earthworks, ” Watling said.

Watling, who was working toward her PhD at the University of Exeter at the time the research was carried out, and her colleagues dug soil samples from holes five feet deep within and outside two geoglyph sites.

They selected Jaco Sá, which features a square earthwork, a circle within a square, and a rectangular embankment between the two, and Fazenda Colorada, consisting of three earthworks, a circle, a square and a double U-shape.

To date the sites, which were dug at various times between the first and 15th centuries, the researchers used charcoal associated with artifacts found at the bottom of the ditches.

The team then analyzed the soil profiles for charcoal, which points to burning activity, and stable carbon isotopes, to provide an indicator of the dominant vegetation types present before, during and following construction of the geoglyphs.

The researchers also analyzed the samples for phytoliths, a type of microscopic plant fossil made of silica that helps identify ancient species. This allowed them to reconstruct millennia of vegetation and fire history around the two geoglyph sites.

It emerged that the bamboo forest ecosystem that exists in the region today was present throughout the past 6,000 years.

Most intriguingly, the investigation revealed that the huge earthworks were created by indigenous peoples in Brazil long before European contact, but their construction showed no signs of deforestation or altering on a grand scale. Rather than clear cutting or burning large tracts of forest, only small, temporary clearings were made for the geoglyph constructions.

“Instead of clearing large areas of forest for long periods, as is the case today, people managed the forest in a variety of successional stages and encouraged tree species that were useful to them, such as palms trees,” Watling said.

She noted that legacies of pre-Columbian agroforestry still exist today within Acre’s remaining forests.

The mysterious earthworks reveal that large populations were able to flourish in Amazonian forests without causing large-scale environmental degradation.

“The study shows that long-term, regional-scale deforestation in the Amazonian rainforests is strictly a modern phenomenon,” Watling said. “Our results are another testimony to the ingenuity and importance of indigenous knowledge for creating more sustainable land-use alternatives in Amazonia.”