[kijin]

Heaps of twisted, rusted steel on the surface is a much more efficient source of iron than iron ore in the ground. Ditto for aluminum, which is easy to handle and recycle once it has been refined (provided you can pick it out of the garbage dump) but terribly energy-intensive to extract from the ground.

Anyone who comes after us will not have much coal and oil to burn, but they'll have orders of magnitude easier access to useful metals. Imagine venturing into the half-collapsed vault of an ancient bank and finding tons of pure gold! There's a lot of fun waiting for the Indiana Jones of the next civilization.