  It is suggested that a large deep underocean (or ice) neutrino detector,
given the presence of significant numbers of neutrinos in the PeV energy range
as predicted by various models of Active Galactic Nuclei, can make unique
measurements of the properties of neutrinos. It will be possible to observe the
existence of the tau neutrino, measure its mixing with other flavors, in fact
test the mixing pattern for all three flavors based upon the mixing parameters
suggested by the atmospheric and solar neutrino data, and measure the tau
neutrino cross section. The key signature is the charged current tau neutrino
interaction, which produces a double cascade, one at either end of a lightly
radiating track. At a few PeV these cascades would be separated by roughly 100
m, and thus be easily resolvable in next generation DUMAND-like detectors.
First examples might be found in detectors presently under construction. Future
applications are precise neutrino astronomy and earth tomography. This paper is
an expanded version of hep-ph/9405296, for publication.
