  The impact of neutrino mixing, refraction and absorption on high-energy
neutrino propagation through a thick medium is studied using the MSW evolution
equation with complex indices of refraction. It is found that, owing to the
mixing with sterile neutrinos, the penetrability of active neutrinos may be
many orders of magnitude larger than it would be in the absence of mixing. The
effect is highly sensitive to changes in density and composition of the matter
background as well as to neutrino energy and mixing parameters. This may lead
to observational consequences in neutrino astrophysics.
