  Neutrino astronomy offers the possibility to perform extra-galactic
observations well beyond the photon absorption cutoff above 50 TeV. Based on
observations of cosmic rays, we already know that astrophysical sources produce
particles with at least a million times more energy than this photon cutoff.
Once discovered, either the nature of the sources themselves or the cross
sections of ultra-high energy neutrinos with terrestrial matter may reveal
exotic physical processes that are inaccessible to modern accelerators. Some of
these processes may be due to as-yet unknown physics at the grand unification
scale or beyond. Neutrino telescopes based on optical techniques currently
operating and under construction have apertures measured in several km^3-sr.
Radio and acoustic detection techniques have been demonstrated in laboratory
experiments and are currently used for instrumentation of apertures 10 to
10,000 times larger than optical techniques for neutrinos above 10 PeV. I
discuss the status of current and proposed neutrino telescope projects based on
these techniques. These telescopes have already ruled out some of the more
exotic predictions for neutrino intensity. The upcoming generation of
radio-based and acoustic-based detectors should be sensitive to cosmic
neutrinos above 1 EeV originating through the so-called GZK process. A
comparison of different neutrino telescopes using a common aperture variable
shows how they are complementary in the trade-off of volume versus threshold. I
include a proposal for how neutrino telescopes should report their
sensitivities to facilitate direct comparisons among them and to allow testing
of neutrino brightness models that appear even after publication of the
experimental results.
