He was last heard from here deep in the Amazon Rainforest.

In 1925, he was on his eighth expedition in the Amazon and he had one objective: to find the ruins of a lost city that he called Z.

The idea was based on rumors that had circulated for centuries that there was once large cities filled with people deep in the amazon.

But Fawcett never found Z or any other city and for decades after his disappearance, experts believe that this rainforest was simply too hostile and too remote to ever have supported cities.

Until recently when scientists began finding these, these ditches and mounds are man-made and they're all over the rainforest.

Now, archaeologists think they finally know the answer to an ancient mystery "Where are the lost cities of the amazon?"

Here was the capital of the empire of the Incas, Cusco. Many fine massive walls built by the ancient inhabitants centuries before the Spaniards came, still stand...

In the 16th century, European colonizers poured into Central and South America, bringing with them soldiers, disease, and plans to conquer it.

They encountered many groups of indigenous people, some of whom had long ago built huge cities like Tenochtitlan founded by the Mexican people Around 1325 A.D.

And Cusco founded by the Incas around 1200 A.D.

These cities were constructed with stone and featured well planned roads and neighborhoods.

The Europeans took over both of them, and by the middle of the 16th century were hearing rumors of another city where a great lord goes about continually covered in gold dust and washes it away at night.

Those rumors eventually became the legend of El Dorado, a city made of gold hidden in the amazon rainforest.

These impressive cities made a golden city seem plausible.

So many Europeans set off in search of Eldorado, but they all failed.

Most ended in starvation, disease, and death.

No one found a golden city, but they did record signs that one could exist spanish explorers claimed to see cities that glistened in white, great quantities of maize and town of disproportionate size and a chief with gold idols and very large towns to such an extent that they were astounded.

Explorations continued over the next 200 years or so, but by the 19th century, the legend of El Dorado had been dismissed as a myth.

It wasn't until the early 20th century that these reports inspired a British explorer to revive the search.

For two decades, Fawcett scoured the amazon for remnants of an ancient city focusing on two areas.

The Western amazon in Bolivia and the southern amazon in brazil, but he couldn't find any of the things the early European explorers observed.

He met indigenous people, but described them in his letters as living in small groups and simple villages and he didn't find any sign of stone ruins.

They seemed to support what had become near consensus among experts at the time that this rainforest was too inhospitable to support large, complex societies, let alone cities.

Modern research estimates that around Fawcett's time, only a couple 100,000 people lived in the entire amazon rainforest.

More racist theories at the time held that these indigenous people were too unsophisticated to build cities and others pointed to the amazon soil which appeared to be too infertile to support necessary large scale agriculture.

But Fawcett was determined to prove cities existed and in the final letter to his wife, Faucet assured her by writing you need have no fear of any failure.

Then he left his camp and was never seen again.

For the next several decades.

The amazon was believed to have always been relatively empty, but it turns out Fawcett was looking in the right place just for the wrong thing.

One clue Fawcett missed was in the soil.

In the 1960s, scientists discovered patches of extremely fertile soil analysis showed it contained much more nutrients than usual.

They called it terra preta or black earth and found lots of it in the amazon, especially along rivers.

Further studies revealed that it was created by human waste or the intentional burning of the forest, which adds nutrients to the soil below.

Terra preta was evidence that large scale agriculture was possible in the amazon.

The next clue Fawcett missed would have also been hard to spot.

In the 1990s , along the Xingu River in brazil, a team of archaeologists led by University of Florida's Dr. Michael Heckenberger made a remarkable discovery.

Working with the local indigenous people, they investigated these very long ditches and after mapping them realized these ditches were signs of a large settlement.

They were remnants of carefully designed walls centered around a plaza and some were roads that lead to more settlements.

In fact, just this part of the amazon about the size of New Jersey was once a network of dozens of settlements that experts believe could have been home to at least 50,000 people between 1250s and 1650 A.D..

These settlements were designed to get the most out of the forest without depleting it.

There were delineated areas for gardens and orchards and deeply forested areas between the settlements used for keeping animals and medicinal plants.

These were the lost cities of the amazon and there were many more.

Over the past few decades, experts have uncovered evidence of large settlements all over the Amazon.

A network of trenches here date back to 200-1200 A.D.,

and suggest settlements that could have supported 60,000 people, which is much bigger than many European cities at that time.

And in Bolivia, scientists recently used satellite technology to reveal remnants of U-shaped buildings on the top of pyramids 22 meters tall.

All these discoveries are leading experts to form a new consensus that the lost cities of the Amazon were once home to millions of people.

Fawcett didn't find large populations of indigenous people because an estimated 80-95% had died from smallpox and measles spread by the first generation of European colonizers between the 16th and 17th centuries.

And he didn't find stone ruins because unlike Cusco and Tenochtitlan, the amazon's indigenous people built with wood and earth, which decompose over time.

What remained was an amazon full of dense vegetation and swaths of infertile soil that appeared to be untouched by humans.

When in fact, humans had been engineering it for centuries, these lost cities show how humans and the rainforest once co-existed.

A relationship we're only just beginning to understand.