Asked by: Malcolm Douglas, Dublin
Stonehenge is at least 4,000 years old and still visible today, and monuments buried nearby could be even older. Most modern buildings aren’t that robust, but some traces would likely remain for at least 10,000 years, even if it was just the magnetic trace of the steel bars inside concrete blocks. When Hong Kong Airport was constructed in the 1990s, the island of Chek Lap Kok was flattened and extended, and the straight edge of its northern coastline will be a clue to our civilisation for tens of thousands of years. Our atmosphere also has high levels of plutonium-239 due to nuclear weapons testing during the Cold War. This isotope only occurs in nature in incredibly small amounts, and will be detectable as a pollutant for at least 250,000 years.
But the most enduring signs of civilisation 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 civilisation for millions of years, as the tunnels fill up with sediment washed down by rainwater to create massive industrial ‘fossils’.