Is Anybody Down There?
Paul Davies: Few physical traces would remain discernible for many millions of years, but if aliens — or their robotic surrogates — paid the solar system more than a flying visit, they may have carried out large-scale projects such as asteroid mining and modification, nuclear processing, or even biotechnology on terrestrial organisms. In principle, the downstream remnants of such activities would still be detectable today. Who knows? There could be an ancient alien technosignature right here on Earth, hiding in plain sight.