My take on the Life After People program's answer to this question ...
First, lots and lots of stuff would be just gone in one thousand years. Most houses would have crumbled to ruin in a century, rubble in among the trees. A ship could fly directly over Manhattan and not notice that those hills and trees and brooks there seem to have a regular pattern; that is, the last skyscraper would fall after only about 250 years, and nature would have covered it all soon after.
.... A huge factor is that all the stuff we've been building from concrete is -steel- -reinforced- concrete. While concrete can last thousands of years, the steel expands and contracts a lot, and so steel reinforced concrete goes to rubble pretty fast.
Second, then ... is that old stuff like the pyramids and the Colosseum and other structures that aren't steel reinforced, well the Life After People show stated that they might still be there in 30,000 years. So yes, aliens that visited the Earth could find structures, but they'd have to look.
Third, and what I thought was the most surprising, is that biological artifacts might last the longest. The show made sure that it mentioned some species of sheep dog that they thought would be still herding sheep thousands of years from now. They NEED the sheep, and the sheep need the dogs. The dogs supposedly would breed true for a long time, because if another breed of dog, or wolf, or coyote would show up, the sheep dog would not say 'hey, lets make puppies!' to the strangers ... instead, their reaction to any strange dogs is always 'Just what are you trying to do, get at my sheep?'
So, none of the stuff that we think is really great will be obvious in a thousand years. But if an alien race came and took a good look, they could tell we were here. But not find out that much about us unless they find something like a time capsule.
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