It pleases me greatly that I have lived long enough to see that modern scientists have come to understand that before it was tropical rainforest the Amazon was a cultivated landscape. A huge complex of cities and farms connected by advanced roads and irrigated by huge, carefully planned water management systems. And occupied by hundreds of thousands (if not millions) of sophisticated and technologically capable people.
That it took the massive deforestation of the Amazon to discover most of this is unfortunate. But learning that it was not an Edenic wilderness offers opportunities for regional planning for the future.
Most likely, the structures may have served as a large solar calendar, built by ancient civilizations more than a thousand years ago. The Rego Grande archaeological site was discovered by the naturalist Emilio Goeldi (1859-1917) at the beginning of the last century, and houses monolithic stones strategically placed in the ground. The site was named after the Rego Grande River that borders it and consists of more than a hundred blocks of granite. 
Like the British Stonehenge, one of the most intriguing Neolithic sites in the world, it must have been special in its time, when it was the scene of ceremonies full of offerings, some astronomical. 
After conducting radiocarbon testing and carrying out measurements during the winter solstice, scholars in the field of archaeoastronomy determined that an indigenous culture arranged the megaliths into an astronomical observatory about 1,000 years ago, or five centuries before the European conquest of the Americas began.
One of the rocks in the area of almost circular shape is a sheet of granite 3m (10ft) high, with an opening in the center with about 30cm (a foot) in diameter, called “Hole Stone” (1).There is another stone aligned in relation to this (2) and demarcating the exact sunrise on the December solstice. Another board shaped rock – which measures 2.5 m (8ft) in height (3) is on a slope such that during the December solstice, the apparent movement of the sun seems to “walk” according to the slope of the rock, as if it “draws the way.” Thus, the projected shadow has the same thickness of the rock and points to the east, while the sun goes down in the west. At other times of the year, the rock shows that the sun is further north, since the sun is at its southernmost position. We can see the perfect alignment between Rock 1 with Rock 2 by sighting through the hole.
Rego Grande and other sites with megaliths have traces of having also been used as cemeteries, another characteristic typical of this type of prehistoric structure. Funeral urns made in the aristé pottery style, marked by red designs on a white background, or dotted with engravings made on clay that was still damp, were found in these places. Pieces of decorated vases, found with the urns, indicate that the dead may have been buried alongside offerings. “The sites with large megaliths must have been reserved for the most important people in the tribe,” says Brazilian archeologist João Darcy de Moura Saldanha. The problem is that pottery in this style has been regularly found in prehistoric sites that have no stone monuments. Common along all of the northern coast of Amapá and in French Guyana, the elaborate aristé pottery stopped being produced after the Europeans arrived in the Americas and, according to Mariana, it cannot be associated with any current indigenous people from the region.
About 10 years ago, after securing public funds to cordon off the stones, Brazilian archaeologists led by Ms. Cabral and Mr. Saldanha began excavating the site, which is shaped roughly like a circle. They soon identified a portion of a river about 3.25km (2mi.) away where the granite blocks may have been quarried.
They also found ceramic burial urns, suggesting that at least part of the Rego Grande site may have been a cemetery, while colleagues from Amapá’s Institute of Scientific and Technological Research discovered that one of the tall stones seemed to be aligned with the sun’s path during the winter solstice.
Their findings, along with other archaeological discoveries in Brazil in recent years — including giant land carvings, remains of fortified settlements and even complex road networks — are upending earlier views of archaeologists who argued that the Amazon had been relatively untouched by humans except for small, nomadic tribes.
Unfortunately, diseases brought by Europeans to the Americas wiped out an estimated 95% of the indigenous population long before explorers ventured into the interior of either North or South America.
Sources:
Pesquisa (Brazilian science journal)
Ceticismo (Brazilian magazine)
Stannous Flouride's Book of Truthiness 