  Live $^{60}$Fe has recently been reported in a deep-ocean ferromanganese
crust. Analysis of the isotopic ratios in the sample suggests that the measured
$^{60}$Fe abundance exceeds the levels generated by terrestrial and cosmogenic
sources, and it has been proposed that the excess of $^{60}$Fe is a signature
of a supernova that exploded near the earth several Myr ago. In this paper, we
consider the possible background sources, and confirm that the measured
$^{60}$Fe is significantly higher than all known backgrounds, in contrast with
the reported abundance of live $^{53}$Mn. We discuss scenarios in which the
data are consistent with a supernova event at a distance $D \sim 30$ pc and an
epoch $t_{\rm SN} \sim 5$ Myr ago. We propose tests that could confirm or
refute the interpretation of the $^{60}$Fe discovery, including searches for
$^{10}$Be, $^{129}$I and $^{146}$Sm. Such a nearby supernova event might have
had some impact on the earth's biosphere, principally by enhancing the
cosmic-ray flux. This might have damaged the earth's ozone layer, enhancing the
penetration of solar ultraviolet radiation. In this connection, we comment on
the Middle Miocene and Pliocene mini-extinction events. We also speculate on
the possibility of a supernova-induced "cosmic-ray winter," if cosmic rays play
a significant role in seeding cloud formation.
