Neutrino Mass Spectrum with $ν_μ$ $\to$ $ν_s$ Oscillations of Atmospheric Neutrinos

We consider the ``standard'' spectrum of the active neutrinos (characterized by strong mass hierarchy and small mixing) with additional sterile neutrino, $\nu_s$. The sterile neutrino mixes strongly with the muon neutrino, so that $\nu_{\mu} \leftrightarrow \nu_s$ oscillations solve the atmospheric neutrino problem. We show that the parametric enhancement of the $\nu_{\mu} \leftrightarrow \nu_s$ oscillations occurs for the high energy atmospheric neutrinos which cross the core of the Earth. This can be relevant for the anomaly observed by the MACRO experiment. Solar neutrinos are converted both to $\nu_{\mu}$ and $\nu_s$. The heaviest neutrino ($\approx \nu_{\tau}$) may compose the hot dark matter of the Universe. Phenomenology of this scenario is elaborated and crucial experimental signatures are identified. We also discuss properties of the underlying neutrino mass matrix.