  We derive constraints on the relic abundance of a generic particle of mass
$\sim~1-10^{14}$ TeV which decays into neutrinos at cosmological epochs, using
data from the Fr\'ejus and IMB nucleon decay detectors and the Fly's Eye air
shower array. The lifetime of such unstable particles which may constitute the
dark matter today is bounded to be greater than $\sim~10^{14}-10^{18}$ yr,
depending on the mass. For lifetimes shorter than the age of the universe,
neutrino energy losses due to scattering and the expansion redshift become
important and set limits to the ability of neutrino observatories to probe the
early universe.
