  We address the possibility that intelligent civilisations that destroy
themselves could present signatures observable by humanity. Placing limits on
the number of self-destroyed civilisations in the Milky Way has strong
implications for the final three terms in Drake's Equation, and would allow us
to identify which classes of solution to Fermi's Paradox fit with the evidence
(or lack thereof).
  Using the Earth as an example, we consider a variety of scenarios in which
humans could extinguish their own technological civilisation. Each scenario
presents some form of observable signature that could be probed by astronomical
campaigns to detect and characterise extrasolar planetary systems. Some
observables are unlikely to be detected at interstellar distances, but some
scenarios are likely to produce significant changes in atmospheric composition
that could be detected serendipitously with next-generation telescopes. In some
cases, the timing of the observation would prove crucial to detection, as the
decay of signatures is rapid compared to humanity's communication lifetime. In
others, the signatures persist on far longer timescales.
