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The article is a few years old but I just read this last night and thought it was interesting. The remains of the earthwork ditches which are 16 meters deep have remnants from pollen stores/fire from 6,000 years ago. And the pre-Amazonians would clear the Forest and area to make their lives more habitable, its kind of a change from the pace that Indigenous people "were in tune" with nature. Anyway, they don't know the purpose of the trenches, but I thought it was at least a partial benefit from all the deforestation. It makes you wonder what other lost cultures are still out there.

http://www.livescience.com/46682-earthen-rings-predate-amazon-rainforest.html