Editor's note: GlobalPost featured this article in Great Weekend Reads, a free compilation of the week's most colorful stories. To receive Great Weekend Reads by email, let us know at editors@globalpost.com.

RIO BRANCO, Brazil — It’s not every day a traveler casually looks out an airplane window and discovers the ruins of an unknown ancient civilization.

“I said, ‘Wow, what am I seeing?’” said Alceu Ranzi, a geographer and paleontologist with the Federal University in the Brazilian state of Acre. “This is not natural.”

What he saw that afternoon in 1999 was a giant, geometrically precise circle carved in the earth. Nearly invisible from the ground, the earthwork had only recently emerged when farmers cleared a tract of forest. The huge structures suggested the area may have been home to many more people than anyone previously believed.

“It’s something new that no one expected,” Ranzi said. “No one expected that a discovery of this level still could have happened in that region.”

Ranzi recruited a team of archaeologists from Brazil and Finland and began searching for more of these earthworks, which he calls geoglyphs.

Their work on the ground in this Brazilian border state has recently received an unlikely boost from space. New Google Earth images this year revealed even more of the formations. The scientists now say they’ve tallied 300 massive shapes — circles, squares, rectangles — scattered across 3,900 square miles.

Ranzi said the size and breadth of the discovery reveals that, some 1,000 years ago, the region was home to a large, complex society. The find has expanded a rapidly growing body of archaeological evidence suggesting that the Amazon — once thought to be largely wild in antiquity — was home to civilizations that may have rivaled those of the ancient West.

“It could be something as important as an unknown Roman empire, or a Mesopotamia,” Ranzi said. “It was completely covered by the forest for six or 10 centuries and now is reappearing.”

Proving this will take decades more work, he said. Scientists are only just beginning to document the size and scope of ancient societies in Amazonia, a region roughly the size of the continental United States. Years after seeing his first geoglyph, Ranzi said he’s only beginning to understand its purpose.

The glyphs are geometrically perfect shapes, usually two to three football fields wide and sketched on earth with giant ditches that average 35 feet across and 3 to 10 feet deep. Soil piled alongside created banks 1.5 to 3 feet high.

They date to between 850 and 1,000 years ago, Ranzi said. So far scientists have found little evidence of human habitation inside the shapes, leading researchers to suspect they were used for ceremonies and kept clean.

And, based partly on the tremendous amount of manpower it would have taken to dig the ditches, researchers estimate that at least 90,000 people might have lived among the 300 glyphs so far identified. Up to this point, scientists have only been able to explore deforested land, so they suspect they’ve documented barely more than a tenth of existing geoglyphs.

The importance of these shapes goes beyond what they reveal about a civilization inhabiting this particular corner of the jungle. They’re part of a rising tide of evidence that’s overturning long-held assumptions about the history of the entire Amazon basin.

Several decades ago, most scientists believed the soil here was too poor and its climate too extreme to support anything more than small groups of hunters and gatherers or small-scale farmers.

That began to slowly change in the 1970s and 1980s, as accumulating evidence suggested that elaborate civilizations, maybe numbering in the millions, had existed along the banks of the Amazon’s largest rivers. Then, in the 1990s, scientists began documenting sites along the southern edge of the Amazon, suggesting that area may have held ancient societies as sophisticated as those of the ancient Western world.

“In many respects they were more complex than classical Greece, or medieval Europe,” said anthropologist Michael Heckenberger, who has spent nearly two decades working near the headwaters of Brazil’s Xingu river, several hundred miles east of Acre.

His team of researchers has described communities housing at least 50,000 people, living in what he’s called garden cities — made up of small and medium residential areas built around central plazas and interconnected by elaborate road systems, aligned with the cardinal directions.

“We also found large defensive earthworks around settlements and indications of pretty complex systems of forest and wetland management,” Heckenberger said.

Heckenberger’s work was featured prominently in the best-selling book “The Lost City of Z.” The non-fiction work recounts the disappearance of world-famous British explorer Percy Fawcett, who vanished into the Amazon in 1925 while hunting for a lost civilization. At the end of the book, author David Grann concludes Heckenberger may have come as close as anyone ever will to uncovering Fawcett’s mythical city of gold.

And he’s not the only researcher revealing a real-life El Dorado in Amazonia.

In Bolivia, a team lead by archaeologist Clark Erickson has found evidence of extensive ancient land management: a landscape marked by causeways, canals, raised fields, ring-shaped ditches and massive earthworks for trapping fish.

Geographically, the earthworks in Acre are roughly contiguous with these two sites, and the archaeological remains have some characteristics in common with those in the other two areas, scientists say.

“Now we know that they were in Acre, they were in Bolivia, they were in the Xingu. Basically all along the southern borderlands of the Amazon we find another group of complex societies,” Heckenberger said. “It clearly represents a much denser population than previously assumed.”

And the research has only just begun. Ranzi’s team is working with the Brazilian government to use a new technology that can map the ground beneath forest has not yet been chopped down.

“Amazon still is an unknown quantity,” said Jaco Cesar Piccoli, an anthropologist and head of the department of social sciences at the Federal University of Acre. “It's likely we’re going to find many more remains of human occupation.”

But he says, even as scientists broaden their search, some ruins are already being destroyed. Roads have been built across many geoglyphs, and cattle graze on and around many more.

“Very little has been done, up until now, to preserve these sites,” Piccoli said. “And there’s still so much left to discover.”