  We examine the possibility that a nearby supernova explosion could have
caused one or more of the mass extinctions identified by palaeontologists. We
discuss the likely rate of such events in the light of the recent
identification of Geminga as a supernova remnant less than 100 pc away and the
discovery of a millisecond pulsar about 150 pc away, and observations of SN
1987A. The fluxes of $\gamma$ radiation and charged cosmic rays on the Earth
are estimated, and their effects on the Earth's ozone layer discussed. A
supernova explosion of the order of 10 pc away could be expected every few
hundred million years, and could destroy the ozone layer for hundreds of years,
letting in potentially lethal solar ultraviolet radiation. In addition to
effects on land ecology, this could entail mass destruction of plankton and
reef communities, with disastrous consequences for marine life as well. A
supernova extinction should be distinguishable from a meteorite impact such as
the one that presumably killed the dinosaurs.
