Footprints of alien technology

If alien civilizations do, or did, exist, their technology will impact their environment. Some consideration has been given to the detection of large-scale astro-engineering, such as Dyson spheres. However, a very advanced technology might leave more subtle footprints requiring sophisticated scientific methods to uncover. We must not overlook the possibility that alien technology has impacted our immediate astronomical environment, even Earth itself, but probably a very long time ago. This raises the question of what traces, if anything, might remain today. I shall consider the possibilities of biological, geological and physical traces, and suggest ways that we might search for them.

► Evidence for alien technology might involve subtle and very ancient physical traces.
► Traces may include environmental disruption, resource depletion or engineered structures.
► Earth and its moon should not be excluded in a search for such traces.
► Markers of radioactive waste, mining and biotechnology might endure for millions of years.