  If muon neutrinos produced in cosmological sources oscillate, neutrino
telescopes can have a chance to detect tau-neutrinos. In contrast to muon
neutrinos the Earth is completely transparent for tau neutrinos thanks to the
short life time of tau-leptons produced in charged current interactions.
Tau-lepton decaysin flight producing another tau neutrino (regeneration chain).
Thus, tau neutrinos cross the Earth without being absorbed, though loosing
energy both in regeneration processes and in neutral current interactions.
Neutrinos of all flavors can be detected in deep underwater/ice detectors by
means of Cherenkov light emitted by charged leptons produced in neutrino
interactions. Muon and tau-leptons have different energy loss features, which
provide opportunities to identify tau-events among the multitude of muons. Some
signatures of tau-leptons that can be firmly established and are background
free have been proposed in literature, such as 'double bang' events. In this
paper we present results of Monte Carlo simulations of tau-neutrino propagation
through the Earth accounting for neutrino interactions, tau energy losses and
tau decays. Parameterizations for hard part and corrections to the soft part of
the photonuclear cross-section (which contributes a major part to tau energy
losses) are presented. Different methods of tau-lepton identification in large
underwater/ice neutrino telescopes are discussed. Finally, we present a
calculation of tau neutrino double bang event rates in cubic km scale
detectors.
