All the coastal areas were out on the Continental shelf.
Few haul their junk uphill to dispose of it, and a really advanced civilisation would have mined their landfills for raw materials, much as we will someday. What scattered bits may be elsewhere were degraded by weathering and oxidation, or destroyed, or scrounged for raw materials by the more primitive technologies to follow.
Seems likely to me, and while people will seldom get everything, the elements will get most of the rest.
The few remnants which survive are likely attributed to other sources or simply not recognized as technologically produced material.
Agreed on the shoreline aspects of your post. In fact, research along the Pacific NW coast for old (5,000 years +) villages was moved offshore and several prospects have been found.
However:
“...and a really advanced civilisation would have mined their landfills for raw materials, much as we will someday.”
We will only start mining our landfills when the more productive natural sources of copper, gold, silver, etc. run out. Why would our natural resources still be around if a previous civilization(s) had to resort to mining their landfills?
I find stuff like this very interesting, but sometimes it doesn’t sit quite right. A more plausable (!!!!?????) explanation would be it was a small civilization that was only advanced because of ET help. It would then not have to have followed “our” model of civilization of increasing in knowledge AND population - but just grow in knowledge. Another theory might be that this junk IS from rockets or some such - but ET rockets. Repair Station Earth, back when the Intergalactic Route 66 was the main route through here 10,000 years ago.
Of course that would require there to be ET’s. Not sure on that one either! But interesting stuff all the same!