  Energy spectra and zenith angle distributions of atmospheric muons are
calculated for the depths of operation of large underwater neutrino telescopes.
The estimation of the prompt muon contribution is performed with three
approaches to charm hadroproduction: recombination quark-parton model,
quark-gluon string model, and perturbative QCD based models. Calculations show
that the larger are zenith angles and water thickness above the detector, the
lower is the energy at which the prompt muon flux becomes equal to conventional
one (``crossing energy'') . For instance, for the depth of the Baikal Neutrino
Telescope and for zenith angle of 78 degrees the crossing energy is about 300
TeV, whereas it is only 8 TeV for the NESTOR depth. Nevertheless, the muon flux
of the crossing energy at NESTOR depth is in order of magnitude lower in
comparison with the Baikal depth.
