I read a book by Alan Weisman titled The World Without Us, which covers this very specific topic. According to it, among the very last man-made structures left after hundreds of thousands – if not millions – of years would be the Channel Tunnel between England and France. Another notable example would be the stone faces on Mount Rushmore, as well as some old steel bridges built in an era when engineers couldn’t yet calculate structural load precisely, so they simply overbuilt everything.