A problem was that the Amazon rainforest was thought of as pristine and untouched. Any people you found living there were, in the big scheme of things, an anomaly - which made logging their homes a lot easier.
But surveys found that there were areas where plant diversity was unexpectedly high, and the soil unexpectedly fertile. There was also another problem that you certainly found complex civilisations in tropics elsewhere, but the urban density was a lot lower compared to Europe.
The area they've looked at are the Amazon lowlands of Bolivia. It's easy to forget that Bolivia has lowlands as a lot of the tourism (and archaelogy) has concentrated on the highlands.
When Prümers & colleagues looked at the data they found two large and complex sites. The architecture is a lot of earthwork. But earthwork doesn't often form neat terraces, complete with a sophisticated system of canals and reservoirs. If you're a fan of pyramids with your ancient civilisations, it has those too.
All this earth needs shifting, and that takes a lot of people
In their article the authors write: "The scale, monumentality, labour involved in the construction of the civic-ceremonial architecture and water-management infrastructure, and the spatial extent of settlement dispersal compare favourably to Andean cultures and are of a scale far beyond the sophisticated, interconnected settlements of southern Amazonia, which lack monumental civic-ceremonial architecture."
Add in that Amazonia is a vast area and largely unsurveyed, archaeologically speaking, and you potentially have an unknown civilisation that would be like finding the Khmer (Angkor Wat etc.) or Shona (Great Zimbabwe) for the first time.
It's an interesting article in itself that you can read at https://doi.org/10.1038/s41586-022-04780-4 (Open Access), but it also is exciting for pointing how much is unknown that remains to be discovered.
- If people can get to the sites, before they're flattened by development.
When you start looking at the Amazon for low-density settlements then things become a lot more puzzling. But the Amazon has been difficult to survey. There are still a lot of trees there. The cleared patches can have damage that makes fragile archaeological clues unreadable.
Heiko Prümers and colleagues have been getting round this with lidar, the laser equivalent of radar, and have been able to use this to survey large areas that wouldn't be possible on foot. And they've found plenty.