[brian_cloutier]

Why would they have left no traces? Dinosaurs lived ~200 M years ago and left enough traces for us to discover them. Wouldn't an industrial civilization 50 mya have left some kind of refined metallic artifacts? Even if _most_ traces are eroded away with time it seems difficult to imagine every trace of a global industrial civilization would disappear.

There also don't appear to have been any spikes in atmospheric carbon dioxide during the relevant period: https://en.wikipedia.org/wiki/Carbon_dioxide_in_Earth%27s_at...

Though maybe you were looking at a data source with better resolution?

[dadoomer]

The answer is that the traces are not super easy to find, depending on how we characterize what traces are in the first place. The question posed is known as the "Silurian Hypothesis":

https://www.scientificamerican.com/article/could-an-industri...