  The ultrahigh energy neutrino cross section is well understood in the
standard model for neutrino energies up to 10$^{12}$ GeV. Test of neutrino
oscillations ($\nu_\mu\leftrightarrow\nu_\tau$) from extragalactic sources of
neutrinos are possible with large underground detectors. Measurments of
horizontal air shower event rates at neutrino energies above 10$^{10}$ GeV will
be able to constrain nonstandard model contributions to the neutrino-nucleon
cross section, e.g., from mini-black hole production.
