A circle of ancient megaliths that has been discovered in the jungle of the Amazon has been challenging the long-held assumption by archaeologists that the inhabitants there were simply tribes of hunter-gatherers. The granite stones, first uncovered in the 1990s, have recently been found to have functioned as an astronomical observatory, much like its more famous sister site, Stonehenge.

The Rego Grande megaliths, also known as the Amazon Stonehenge, is a complex of 127 granite stones found in Brazil’s Amapá state that range in size up to 4 meters (13.1 feet) tall, arranged in a circle roughly 30 meters (98.4 feet) across. It was first discovered by rancher Lailson Camelo da Silva in the 1990s, while he was clearing the jungle for a new pasture. “I had no idea that I was discovering the Amazon’s own Stonehenge,” remarks Mr. da Silva. “It makes me wonder: What other secrets about our past are still hidden in Brazil’s jungles?”

Archeological excavation at the site started in 2005, uncovering pottery and burial pits that dated back between 500 to 2,000 years. While researchers are still not sure as to the extent of what the site was used for, recent observations by archaeoastronomers have found that the larger megaliths appear to be aligned with the winter solstice. The alignment of the stones indicate that they were arranged to correspond with alignments from 1,000 years ago.

The sophistication involved with the planning of such a site adds to a growing body of evidence that the Amazon Basin may have been home to indigenous cultures that were larger and more sophisticated than what previous researchers have assumed, with a pre-Columbian population of as many as 10 million people.

“We’re starting to piece together the puzzle of the Amazon Basin’s human history, and what we’re finding in Amapá is absolutely fascinating,” explains the site’s lead archaeologist, Mariana Cabral, with the Federal University of Minas Gerais.