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The Search for Extraterrestrial Intelligence came up empty. So we lowered our sights and began the Search for Extraterrestrial Life. This too was inconclusive. Chemical signatures in spectra could indicate life, but could also be inorganic in origin.
Then, one day, we found a signature that was unmistakably from a civilization that had recently poisoned its world to extinction.
Annoyingly, we could still not answer the question of whether we were currently alone in the universe.
