In a stretch of the Bolivian Amazon known as the Llanos de Moxos, the sultry port of Loma Suárez takes its name from a notorious rubber baron who built a mansion and ranch beside a loma (hill) overlooking the Ibare River. During the late 19th and early 20th Centuries, Nicolás Suárez and his brothers were among the richest – and most ruthless – people in Bolivia, ruling over a vast swathe of the Amazon Basin with terrifying violence, according to my guide Lyliam González. "They owned everything around here," she said.
The eponymous hill is now topped with a mausoleum for one of the brothers, Rómulo – but I was more interested in the grassy mound itself. Around 10m in height, with a dirt path and a cluster of trees at the base, it appeared natural and nondescript. Yet it is actually man-made, one of thousands of earthworks built by remarkable but little-known ancient societies.
The Amazon prior to the arrival of Europeans in the Americas in 1492 is commonly depicted as a pristine wilderness dotted with small, simple communities. The Llanos de Moxos (or "Mojos") elegantly rebuts this notion. Spanning 120,000 sq km of tropical savannah, rainforest and snaking waterways in north-east Bolivia, the region – which is roughly the size of England – has been inhabited for 10,000 years, initially by hunter-gatherer communities. Around 1000 BCE, more complex societies started to develop.
In response to the highly challenging environment – including dramatic seasonal floods – these people built networks of earthwork structures: hills; elevated residential and ceremonial platforms; raised fields to protect against rising water levels; plus causeways, canals, aqueducts and reservoirs. Pioneering US archaeologist Kenneth Lee – who first visited the region in the 1950s while working for Shell and ended up dedicating his life to the study of the earthworks (a museum in the nearby city of Trinidad, the Museo Etnoarqueológico Kenneth Lee, now bears his name) – estimated there were as many as 20,000 earthworks, with the largest villages home to 2,000 people or more.
Loma Suárez is one of thousands of earthworks built by the ancient societies of the Llanos de Moxos (Credit: Shafik Meghji)
Unlike the Inca or Maya, there's no single name for the ancient earthwork-builders of the Llanos de Moxos. The few academics who study them tend to use awkward collective terms, such as "pre-Hispanic" or "pre-Columbian", while individual groups – such as the Baures or Casarabe cultures – have been named after modern-day villages or towns.
But in recent decades the earthwork-builders have come under greater study by archaeologists, whose findings have transformed our understanding of the Amazon. Recent research suggests that for more than 2,000 years, the Llanos de Moxos was home to far more people – perhaps as many as a million – and far more sophisticated societies than previously thought. Despite lacking vital resources such as local sources of stone and domesticated animals, these societies completely reshaped their surroundings, building an array of earthwork structures for homes, agriculture, religious ceremonies and burial grounds that enabled them to thrive in a landscape that even today can prove highly testing.
This construction work involved the "mass movement of soils, transformation of local topography, soil enrichment, and change in vegetation composition", according to University of Pennsylvania archaeologist Clark L Erickson in his research paper Amazonia: The Historical Archaeology of a Domesticated Landscape. Artificial canals and causeways provided transport and communication links, helping to not only mitigate the damage from seasonal floods but actively manage the water levels. Lagoons and weirs were created to aid fishing, while other earthwork formations were designed to drive wild animals on to designated patches of dry land, where they could be more easily hunted.
Although many of these structures were abandoned in the 15th Century – possibly because of conflict, drought or famine – and have since been swallowed by the jungle, some are still occupied by indigenous communities (the descendants of the earthwork-builders), while others have been subsumed into towns and ranches and a few have been protected through conservation projects.
Map showing the Llanos de Mojos savannah and the Casarabe Culture area (Credit: H Prümers/Deutsches Archäologisches Institut)
To learn more about them, I arranged an overnight stay at Chuchini, a nearby nature reserve and ecolodge on another man-made loma. At Loma Suárez's dock, I met guide Efrem Hinojosa – whose parents founded Chuchini half a century ago – and boarded a motorboat for a short journey north along the Ibare River.
"My parents created the [Chuchini] reserve in 1973 after they learned about the area's archaeological and environmental significance," said Hinojosa, as we cruised past lurking camains, their prehistoric snouts poking above the surface of the river.
After 15 minutes, we turned into a narrow channel slicing through the dense green riverbank. Lined with reeds and spindly trees, it was also an ancient earthwork, a canal built 1,000 years ago or more, explained Hinojosa. Shortly afterwards, we emerged into a shimmering lagoon overlooked by a squat green hill ringed with rainforest and patrolled by a pair of yapping dogs.
The name 'Chuchini' means 'Den of the Jaguar', one of around 100 mammal species you find here
Hinojosa's wife Miriam showed me around the loma, which was far larger than the one occupied by the Suárez mausoleum. The centre of the flat, grassy summit was home to the couple's ecolodge – a set of spick-and-span guest rooms, a breezy semi-open dining area, plenty of hammocks, a small playground and a football pitch – while walking trails led off into the surrounding jungle, which echoed with birdsong.
"The name 'Chuchini' means 'Den of the Jaguar', one of around 100 mammal species you find here," Miriam explained. "There are also more than 300 species of birds." More than 1,500 artefacts – notably, finely worked ceramic pots, urns and figurines produced by the earthwork-builders – have been excavated in the reserve and more are being discovered all the time (including, recently, an adult skeleton).
More than 1,500 artefacts, including ceramic figurines, have been excavated in Chuchini nature reserve (Credit: Chuchini)
The family's foresight protected Chuchini from the deforestation, poaching, ranching and commercial agriculture that has destroyed much of the region. Today, the reserve relies on tourism: locals come for the day to splash around in the lagoon, lounge in hammocks and wander the trails; while foreign travellers tend to stay for a few nights, often taking part in volunteer programmes. A qualified vet, Hinojosa also runs a wildlife rehabilitation centre. Among his patients that day were racoon-like coatis, several monkeys and a pair of handsome toucans, their orange-and-yellow beaks so unnaturally bright I mistook them for plastic replicas.
If you look at some of the earthworks from the air, they look like human or animal figures
After I spent the day swimming and hiking, Hinojosa took me around Chuchini's small museum, which was packed with artefacts offering a tantalising glimpse at the cultures, beliefs and rituals of the people who once lived here. There were ceramic figurines, including a one-legged man with a protruding belly button and the torso of a woman who appeared to be wearing a spotted bikini. Two large funerary urns contained human remains, including a full set of teeth. Other pots were decorated with geometric patterns that some speculate represent ancient maps. "If you look at some of the earthworks from the air, they look like human or animal figures," said Hinojosa. "Like Peru's Nazca Lines."
Although archaeological interest in the Llanos de Moxos is relatively recent – the first excavations were carried out in the 1910s, but the extent of the earthworks only started to become apparent half a century later – the region has long captivated outsiders. In his 1609 book Royal Commentaries of the Incas, Spanish-Inca historian Garcilaso de la Vega wrote about a 15th-Century Inca expedition into an Amazonian province called Musu – thought to be the Llanos de Moxos – where they found a "great many warlike people" who, while "delighted to be… friends and confederates", refused to submit to Inca rule.
This account helped to inspire the legend of El Dorado, a city of immense wealth lost in the jungle. Over the following centuries, countless expeditions headed into the Amazon in search of these fabled riches. None succeeded, many people lost their lives and the notion that advanced societies may once have existed in this part of the world was widely dismissed.
But in recent decades, studies of the Llanos de Moxos have shifted this view. They demonstrate how these societies sculpted, tamed and exploited the landscapes around them, creating – to quote Charles C Mann, author of the book 1491: The Americas Before Columbus – "one of the largest, strangest and most ecologically rich artificial environments on the planet".
This year, cutting-edge research shed new light on the earthwork-builders. In May, a group of archaeologists and scientists from Germany and the UK published the results of a survey that used laser-scanning technology to examine the south-eastern Llanos de Moxos. In a paper in Nature, they describe a form of "low-density urbanism" that bears comparison with contemporaneous – and better known – Andean societies, such as the Tiwanaku empire, whose eponymous capital now lies in ruins near Lake Titicaca. (A strong influence on the Inca, the Tiwanaku once dominated a vast area spanning much of modern-day Bolivia, southern Peru, north-east Argentina and northern Chile.)
The team found several sites built by the Casarabe culture (circa 500-1400 CE), including a pair of large settlements: the construction process for the bigger of the two involved the movement of a staggering 570,000 cubic metres of earth – enough to fill 228 Olympic-sized swimming pools. The settlements featured stepped platforms that were topped, in some cases, with 22m-tall pyramids. They were also connected to neighbouring communities by raised causeways stretching for several kilometres and surrounded by canals, reservoirs and artificial lakes.
A recent lidar survey has shed new light on sites built by the Casarabe culture (Credit: H Prümers/Deutsches Archäologisches Institut)
Heiko Prümers, an archaeologist at the German Archaeological Institute and a co-author of the study, told Nature that the complexity of these sites is "mind-blowing".
The scale and sophistication of the Casarabe culture and their counterparts are even more impressive when you consider the geographic and climatic challenges in the Llanos de Moxos. They also faced extreme weather phenomena – into which I got a first-hand insight. Overnight the heat and humidity ramped up before being broken by an almighty storm so powerful it rattled the walls of my guest room. It was a surazo, said Miriam over breakfast, frigid polar winds that periodically blow up from Antarctica, plunging temperatures and resulting in great downpours.
On the boat back to Loma Suárez – numb from the cold, lashed by raindrops resembling hailstones – I felt a fresh sense of respect for the ancient societies of the Llanos de Moxos, who not only carved out an existence here, but managed to flourish.
Shafik Meghji is the author of Crossed off the Map: Travels in Bolivia
BBC Travel's Lost Civilisations delves into little-known facts about past worlds, dispelling any false myths and narratives that have previously surrounded them.
Join more than three million BBC Travel fans by liking us on Facebook, or follow us on Twitter and Instagram.
If you liked this story, sign up for the weekly bbc.com features newsletter called "The Essential List". A handpicked selection of stories from BBC Future, Culture, Worklife and Travel, delivered to your inbox every Friday.