There is actually a lot of evidence of the biblical flood about 4.5k years ago
and there is evidence that many ancient structures and civilisations were pre flood by over 1000years
as we know many underwater cities and evidence of civilisation
a lot of archaeological evidence of ancient civilisations over 5000 years ago this was the time Adam was still alive as he lived to 900+
the fallen angels and their hybrid offspring half demon half human were on earth for an unknown time before the flood maybe around 1000years, but probably only towards the end of that did they really get going with the wonders of the ancient world, then the flood destroyed most of it.
much of it remained like the many many pyramids around the world
check out the mysterious ancient Egyptian like stuff going on in the hidden parts of the Grand Canyon which experts think are over 5000yeats old