When the Conquistador Francisco de Orellana entered the remote Rio Negro region of the Amazon basin in 1542, he was searching for one thing -- a city of gold. Though he never found gold, he did find what is now believed to be the lost city of El Dorado and something perhaps even more valuable.

Wide causeways running in perfectly straight lines for lengths of 50 miles or more across huge engineered agricultural landscapes, connecting cities with populations in the hundreds of thousands. This is what Orellana reported upon his return to Spain. But subsequent explorers could never find the legendary agrarian metropolis, and most dismissed his claims as fantasy.

Even modern anthropologists dismissed the El Dorado legend for one good reason -- soil fertility, or lack thereof. The Amazon basis is notorious for its highly eroded and infertile soils, and until very recently it was impossible to consider that this soil could support such a large and extensive population. 

But the Lost City also contained a lost secret. A discovery in 2002 by scientists at Duke University proved Orellana right, and the discovery is documented in a 45 minute BBC video called The Secret of El Dorado.

Thanks to the Replant the Rainforest campaign running this month, I came across this great video which locates the great complex of cities that spanned hundreds of miles and housed as many as a million people. And the secret?

Biochar. A method for burning agricultural waste under oxygen-deprived conditions. The result is solid, sequestered carbon called Terra Preta which enabled the inhabitants of the lost city to produce huge amounts of produce without depleting the surrounding rainforests.

The biochar is able to stimulate the growth of friendly fungi which increases soil volume, stability and fertility. It also sequesters carbon... permanently, making it a candidate for true, "save the world" technology.

Biochar has its critics, who fear that a modern version of the ancient technology could result in a new "biofuel craze" which would only serve to further erode precious rainforest lands (remember when palm oil plantations were considered a biofuel panacea?) But if engineered properly with strong conservation measures in place, biochar could at least turn out to be a part of the solution -- sequestering excess CO2 from the atmosphere while giving developing nations a method for sustainable agriculture that does not rely upon deforestation.

Though Orellana never found his treasure trove of gold, he did ultimately point us to something more valuable -- proof that biochar is a key to supporting dense human populations sustainably and for the long term.