Amazonian Dark Earth, or terra preta do indio, is a bit of a mystery. This rare rainforest soil holds two and a half times the carbon content of normal soils, but no one is entirely sure as to how the ancient tribes of the Amazon actually created the stuff. If the secret can be cracked, we would have access to a carbon sequestration method that would take biofuels from being carbon neutral to carbon negative – in other words, actually pulling out more carbon from the ecosphere than is released by the use of the fuels produced. And the more carbon it stores, the better the crops grown in it perform – any eco-project with a positive feedback loop has got to be worth looking into.