I suspect that this is part of the cycle of civilizations throughout all that lost and forgotten history of this planet. My personal belief is that everything you see around this world will soon enough for many living today will witness first hand exactly how the cosmos treats every planet in the universe(s). In short this is not the first time hominids of some ilk have built great modern technological marvels with the same issues we are experiencing today that ripped them apart... in part because the cosmos can literally drive us crazy before it unleashes all its physical splendors at us. The Earth's Cataclysm Cycle is real.... and it is repeating like clockwork on a scale we have a difficult time recognizing.

If you want to know where the evidence of such things might be... it was almost completely swept away into the seas. That is where all we have built over the last several centuries will end up save for that which is at a high enough elevation built with the correct materials.