[ilaksh]

It's obvious that human history is very short but it's less obvious that all of our buildings and trash will be so hard to find underground in five million years or so.

Wouldn't there at least be some places where ruins or landfills were largely exposed? And aren't there lots of materials that would stand out since they degrade slowly?

For example, what is going to happen to all of the concrete and steel in Manhattan in the course of five million years? Will it really be compressed to a thin layer that is barely noticeable or something?

[ncmncm]

It will all wash away, sand grain by sand grain. You can see the erosion of stone temples even a thousand years old, made of harder stuff than concrete.

[ilaksh]

The steel will wash away?

[ncmncm]

It will rust first, and the rust will wash away. The glass will wash away. Water conquers all.