I remember an episode of "Life Without People", which was a cool series about how the man-made environment would decay if people were suddenly to disappear. It was amazing how relatively quickly our technological achievements would crumble given enough time and neglect. Nature slowly but surely reclaims the land. It was surmised that after 10,000 years, nothing made by man would survive. If aliens were to land after 10,000 years of man's absence there would be no trace of us, except...maybe...
the pyramids of Egypt, which would be half buried and eroded by the sands of the desert...
Mount Rushmore...
and Hoover Dam.