Slowly but surely the full exent of this society is been understood. What they continue to miss is that terra preta soils allow population densities similar to rice culture. We are looking at an environment quite able to support tens of millions.
Who lived there?
The rest is mostly a mystery. So is the fate of these civilisations. Their population peaked about a thousand years ago, and numbers were already falling when Europeans arrived, at which point they nose-dived. We don’t know what caused that initial decline or why many of the societies changed drastically in the 500 years before Europeans arrived. Around this time, the geoglyph-building people seem to have been replaced by people building circular villages – unless they were the same people who changed their practices.
The population could have fallen because the societies reached an ecological limit. Or they could have been fighting – there seems to have been an increase in the number of defensive structures such as palisades around that time.