It would take a long time for all evidence of human existence to vanish after humans disappear. Some traces of human civilization would likely remain for at least 10,000 years. Our atmosphere also has high levels of plutonium-239 due to nuclear weapons testing during the Cold War, which will be detectable as a pollutant for at least 250,000 years. The most enduring signs of civilization will probably be deep mines in hard rock, such as South African gold mines and Australian lead mines. Here, visiting aliens would be able to see signs of our civilization for millions of years.
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