Expected muon energy spectra and zenithal distribution deep underwater

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 a perturbative QCD based model. Calculations show that the larger are zenith angles and water thickness above the detector, the lower is the energy E' ("crossing c energy") at which the prompt muon flux becomes equal to conventional one. For instance, for the depth of the Baikal Neutrino Telescope and for zenith angle of 78 the crossing energy is about 300 TeV, whereas it is only 8 TeV for the NESTOR depth. Nevertheless, the muon flux for E = E' at c NESTOR depth is in order of magnitude lower in comparison with the Baikal depth.