A new study suggests that while a major asteroid or comet impact can destroy all life on the surface of a planet, life deep underground can survive and even flourish. Such a theory wouldn't even have been advanced a few decades ago because we were unaware life existed in the rocks deep inside Earth. Now it's clear even miles under the surface bacteria and microbes thrive by extracting nutrition from rock, for example.
The study points out that the underground environment is extremely stable, and while an impact and subsequent earthquakes could disturb that environment, it would fairly quickly return to normal. An impact's heat would sterilize the upper layers of the world, but it could also mix nutrients with rock and drive them deep beneath the surface, thus enriching underground niches for life.
Again, we are left with the impression that once life has a foothold on a world, eradicating it might well require a deliberate decision and a thoroughgoing and determinedly executed strategy.
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