Dark Earth that is....
Twisted Spinster links to some cool articles here, here, here, here, here and here on the dark earth regions of the Amazon. They point to the notion that The Amazon basin was the home to a thriving civilization that had made impressive advancements in agriculture.
The early reports by explorers about large communities farming in the Amazon are reminicent of La Salle reports of large communities along the Mississippi. Years later subsequent explorers found little evidence. It now seems that both cultures were wiped out unintentionally by diseases and having little save wood buildings left little trace.
The Dark Earth itself is facinating as it could have real and humanistic applications for increacing crop yield in the near future.
In the longer term, understanding it might even someday aid terraforming efforts on Mars.