  Ultra High Energy (UHE) Tau neutrino may lead to a very peculiar imprint in
future underground $Km^3$ detectors in water and ice as well as in air: rarest
secondary tau tracks and decay which may exceed the muon ones. Indeed
Bremsstrahlung at high energy lead to longer tracks for heavier leptons.
Radiation lenght grows nearly with the square of the lepton mass. Indeed
electrons are too light and their trace in matter (liquid or rock) is
negligible (tens of centimeters); muons are much better observed, while tau are
too short life time and too short range to be found. However, because
relativistic time expansion, UHE tau traces in matter, above $10^{17} eV$, are
relativistically boosted overcoming the corresponding muon tracks, already
bounded by bremsstrahlung logarithmic regime. The tau crossing for Kms in water
or ice may be confused with common muon tracks; their tau decay may be
misunderstood as muonic catastrophic bremsstrahlung interactions. To economize
UHE tau discovery, we suggest to look the tau decay in air into the deep
valleys mountains, like Canyons or deep in excavation mines where horizontal
air showers induce fluorescent or Cerenkov lights. The mountain valley width
screens from horizontal secondary muons. The valley height increases the solid
angle view. The horizontal air Kms-size gap offer a strong discriminator to
filter UHE muons against tau. Tens event a year at PeV (W resonance peak)
energies in $Km^3$ excavation gap should be observable . Hunting air shower in
the night toward high mountains in Canyons or in a deep excavation may be the
best and cheapest way to discover UHE neutrinos, either born by electron anti
neutrino scattering on electrons at PeV energies, or by direct tau neutrino
possibly relic of muonic flavour oscillation even at EeV energies.
