Deep in the Amazon rainforest, archaeologist Mark Robinson was buried knee-deep in treasure.
Along with an international team of scientists, Robinson was part of an expedition to a remote patch of forest in Iténez, in northwestern Bolivia, near the border with Brazil.
Getting there was not easy at all. To avoid a 10-hour boat trip, they took a flight to the nearest village, Versalles.
The trip was scary. The plane had to fly in circles over a grass runway to avoid landing on a herd of grazing animals.
Then came a long walk through dense rainforest, past gnarled roots and armies of devastating ants.
“It’s hot, it’s humid, you get stung constantly,” says Robinson, professor of archeology at the University of Exeter, in the United Kingdom.
But the trip was worth it. The researchers’ mission was important. They were looking for the Amazonian terra preta, sometimes called “black gold.”
This layer of coal-black soil can be up to 12 feet thick. It is found in isolated stretches, spread throughout the Amazon basin.
Terra preta is intensely fertile, rich in decomposing organic matter and nutrients such as nitrogen, potassium and phosphorus, essential for agriculture.
But unlike the thin, sandy soil of the rainforest, this layer was not deposited naturally. It is the result of the work of human beings who lived a long time ago.
The nutrient-rich soil is a relic of a very different time, when indigenous groups established a thriving network of settlements throughout the rainforest.
In January 2024, scientists announced the rediscovery of a long-forgotten “garden city.” Hidden beneath forest foliage in Ecuador’s Upano Valley, they found a 2,000-year-old urban center — complete with plazas, streets and ceremonial platforms.
The discovery raised questions about the possibility of other ancient settlements hidden in the Amazon. And this is where black earth comes in.
It is believed that the garden city was only able to support so many people due to the region’s fertile volcanic soil. But in other parts of the Amazon, indigenous communities depended on terra preta to increase their productivity.
Now, there is growing interest in the lessons their ancient methods can teach today’s societies, whether to improve agricultural production or combat climate change.
Surrounded by the scents and sounds of the rainforest in Versalles, a remote location in the Bolivian Amazon, Robinson talks about the temptation to imagine he is in a wild, untouched region. But it’s not true.
“The more we discover, [mais claro fica que esta] it’s not necessarily primary forest,” he explains. “Everywhere we looked, as much as the travel might have been really difficult for us and the place seemed very remote, we simply found evidence of ancient communities everywhere.”
In 2017, research revealed that trees domesticated by pre-Columbian humans are five times more likely to be dominant in the Amazon, compared to those of natural origin. And the closer we get to ancient settlements, the greater the number of these species.
Although today many of the Amazon’s indigenous communities have disappeared (swept away by European colonizers and the diseases they brought), the agricultural practices of the original peoples continue to shape the rainforest.
And another fundamental element of this occult influence is the black earth of the Amazon, found in the forest everywhere.
“That’s what fascinates — it really is pan-Amazonian, we are finding [a terra preta] everywhere,” says Robinson.
This precious layer of soil contains a potent combination of inorganic materials such as ash, clay, bones and shells, as well as organic material such as food waste, manure and urine.
It is at once a treasure trove of ancient waste, hugely exciting for archaeologists like Robinson, and a functional part of the Amazon soil.
Terra preta continues to enrich the rainforest and allow indigenous communities to plant in those places to this day.
“It really is a gold mine,” exclaims Robinson.
Alongside fossilized seeds and pottery artifacts from millennia ago, there are microscopic indications of what the rainforest might have looked like thousands of years ago.
One example is fecal spherolites — tiny crystals found in animal dung, which indicate the types of animals that used to roam the region in the past and defecate there.
Black earth first aroused the interest of Europeans in the 1870s, when several scientists at the time independently observed black layers of soil, which contrasted with the light or reddish earth around them.
An ancient explorer described it as “fine black clay.” He noted that “scattered about it everywhere we find fragments of Indian pottery, so abundant in some places that they almost cover the ground.”
But the way in which the Amazonian terra preta was created is surrounded by a certain mystery.
Scientists have questioned whether this type of soil was produced by accident (as a byproduct of waste disposal by generations of indigenous people) or by an intentional process of enriching the rainforest, making its soil more suitable for agriculture.
In 2023, an international team of scientists addressed this question. Combining an analysis of the structure and composition of terra preta with observations and interviews with the Kuikuro indigenous community (in the southeastern Amazon, central Brazil), the researchers concluded that these layers of soil were, in fact, produced on purpose.
The age and distribution of these soil deposits tell the story of the rise and fall of ancient indigenous civilizations across the Amazon.
While the oldest layers of this black soil are about 5,000 years old, “we observed much more [evidências da produção de terra preta] about 4,000 years ago,” Robinson explains. “There’s a lot more activity, a lot of cultural changes.”
Peak production was only reached around 2,000 years ago, according to the archaeologist. This is the average age of black deposits found across a wide area of the Amazon basin.
At that time, communities were larger and formed vast networks. But the settlements where people produced terra preta were typically not on the same scale as the recently discovered city in Ecuador.
One reason may be the power of the Amazon’s terra preta itself.
In the environment of an abundant forest habitat, enriched by indigenous people with everything they needed — fruit trees and fertile soil for cultivation — Robinson believes that people may not have needed to engage in large-scale agriculture.
“That’s why, [é possível que] you don’t really need the additional hierarchical level [que tende a se desenvolver nos assentamentos maiores]”, he explains.
But about 500 years ago, something clearly went terribly wrong. “That’s when we really see the fall [da produção da terra preta]”, according to the professor.
This reduction is believed to be one of the consequences of Christopher Columbus’ arrival in South America on August 1, 1498. When the Genoese navigator unfurled the red and yellow flag of Spain on the Paria peninsula in Venezuela, he marked the beginning of a “great mortality”.
It is estimated that 56 million indigenous people were killed across the American continent by the year 1600. There were so many deaths that the Earth’s climate cooled.
Although most of the ancient inhabitants of the Amazon disappeared long ago, their legacy remains.
The curious thing is that not all the black earth they left behind has the same composition. In fact, it varies widely depending on the specific ingredients used in different locations.
“But the basic mechanism of soil creation and enrichment appears to be similar,” says Robinson. “They [os povos originários] invest directly in the soil, using their own waste products.”
The soil base is mainly composed of food waste, with the addition of feces and charcoal. And this last ingredient is what attracts more and more attention.
The black earth of the Amazon is not only extraordinarily rich in nutrients. It also acts as a powerful carbon siphon.
Terra preta contains up to 7.5 times more carbon compared to nearby soil.
As terra preta accumulates, carbon becomes trapped underground, where it remains stable for hundreds of years. It is retained, which delays its entry into the atmosphere.
We don’t know for sure why carbon from the Amazon’s terra preta behaves this way. Scientists suspect there is some relationship with “black coal”, also known as “biochar”, or biochar.
This basic ingredient is made from organic material that has been transformed into almost pure carbon under high temperatures in the presence of low levels of oxygen.
The process emits less carbon dioxide than charcoal production and generates a thin, brittle black product, which is found in terra preta throughout the Amazon region.
Now, companies are trying to capitalize on this ancient method. The goal is to help farmers improve their soil while combating climate change.
One example is the company Carbon Gold, which produces biochar for use as an organic planting aid, without the use of peat.
Founded in 2007 by the creator of a chocolate brand, the company based its methods on Mayan cocoa producers in Belize, Central America. They have been using biochar for millennia.
In addition to retaining carbon, “biochar improves structure, aeration, water and nutrient retention capacity”, which can support healthy plant growth, according to Sue Rawlings, managing director of Carbon Gold.
She says that her clients currently include organic producers, gardeners, sports stadiums, English Premier League football clubs, important golf courses and turf tracks, as well as British royal gardens and parks.
Robinson believes that copying the methods of the ancient indigenous people of the Amazon will be essential for future generations. He highlights predictions that, by 2050, around half of the world’s population will live in the tropics, with large numbers migrating to tropical forests.
“I think it is essential to find ways for communities to be more sustainable,” according to him.
“And we can learn about this from the past. I think we’re on the verge of understanding this point.”
This text was originally published on BBC Future and BBC News Brasil.