To find it, you have to go digging in rainforests, and to the untrained eye, it does not seem special at all - just a thick layer of dark earth that would not look out of place in many gardens. But these fertile, dark soils are in fact very special, because despite the lushness of tropical rainforests, the soils beneath them are usually very poor and thin. Even more surprising is where this dark soil comes from.
'You might expect this precious fertile resource to be found in the deep jungle, far from human settlements or farmers,' says James Fraser, who has been hunting for it in Africa's rainforests, 'But I go looking for dark earth round the edge of villages and ancient towns, and in traditionally farmed areas. It's usually there. And the older and larger the settlement, the more dark earth there is.'
Such findings are overturning some long-held ideas. Jungle farmers are usually blamed not just for cutting down trees but also for exhausting the soils. And yet the discovery of these rich soils - first in South America and now in Africa, too - suggest that, whether by chance or design, many people living in rainforests farmed in a way that enhanced rather than destroyed soils. In fact, it is becoming clear that part of what we think of as lush pure rainforest is actually long-abandoned farmland, enriched by the waste created by ancient humans.