  Numerous Earth-destroying doomsday scenarios have recently been analyzed,
including breakdown of a metastable vacuum state and planetary destruction
triggered by a "strangelet'' or microscopic black hole. We point out that many
previous bounds on their frequency give a false sense of security: one cannot
infer that such events are rare from the the fact that Earth has survived for
so long, because observers are by definition in places lucky enough to have
avoided destruction. We derive a new upper bound of one per 10^9 years (99.9%
c.l.) on the exogenous terminal catastrophe rate that is free of such selection
bias, using planetary age distributions and the relatively late formation time
of Earth.
