http://www.bbc.co.uk/news/science-environment-31467619
There have been a few stories over recent years about finds of earthworks suggesting that the pre-contact Amazon basin was more densely populated than previously thought. Now the idea is to survey the region using drones to try to quantify the scale of occupation.
For earlier reports, try
http://news.nationalgeographic.com/news/2008/08/080828-amazon-cities.html
http://news.nationalgeographic.com/news/2010/01/100104-amazon-lost-civilization-circles/