"I'm always amazed that they were able to build a civilization in such a thick jungle."
There is a theory that the jungle only exists today because of the cultivation of plants and leaving of waste by humans in the area previously. They say that the building up of human and livestock waste is what made it so fertile. I have also read that they had a different concept of farming than western civilization. They farmed with many different crops planted together, so that say one that gave back more nitrogen into the soil could fertilize other plants.
It is posited that this is why the jungle is so thick in parts of south america. It was somewhat engineered that way by man.
"Terra preta (black earth), which is distributed over large areas in the Amazon forest, is now widely accepted as a product of indigenous soil management. The development of this fertile soil allowed agriculture and silviculture in the previously hostile environment; meaning that large portions of the Amazon rainforest are probably the result of centuries of human management, rather than naturally occurring as has previously been supposed."