> Terra preta [...] is a type of very dark, fertile artificial (anthropogenic) soil found in the Amazon Basin. [...] Terra preta owes its characteristic black color to its weathered charcoal content,[2] and was made by adding a mixture of charcoal, bone, broken pottery, compost and manure to the low fertility Amazonian soil. A product of indigenous soil management and slash-and-char agriculture,[3] the charcoal is stable and remains in the soil for thousands of years, binding and retaining minerals and nutrients.[4][5] [...] Terra preta soils were created by farming communities between 450 BCE and 950 CE.[10][11][12] Soil depths can reach 2 meters (6.6 ft). It is reported to regenerate itself at the rate of 1 centimeter (0.4 in) per year.[13]
First paragraphs on wikipedia: https://en.wikipedia.org/wiki/Terra_preta
Hancock has a gift for pointing out deeply fascinating aspects of human history. His personal conclusions based on things he brings up include things like advanced, pre-historic civilizations. Regardless of whether you care for those kinds of claims, I think it's almost undeniable that the tidbits he brings up in his discussions, like terra preta, are a real treat to read about. For any fan of history or intellectually curious person, I'd recommend at least watching a video or two of his on youtube, just as a starting point if nothing else, and going from there to read about the various things he brings up.