  After a short review of the ultrahigh energy cosmic ray puzzle - the apparent
observation of cosmic rays originating from cosmological distances with
energies above the expected Greisen-Zatsepin-Kuzmin cutoff 4x10^{19} eV - we
consider strongly interacting neutrino scenarios as an especially interesting
solution. We show that all features of the ultrahigh energy cosmic ray spectrum
from 10^{17} eV to 10^{21} eV can be described to originate from a simple
power-like injection spectrum of protons, under the assumption that the
neutrino-nucleon cross-section is significantly enhanced at center of mass
energies above ~ 100 TeV. In such a scenario, the cosmogenic neutrinos produced
during the propagation of protons through the cosmic microwave background
initiate air showers in the atmosphere, just as the protons. The total air
shower spectrum induced by protons and neutrinos shows excellent agreement with
the observations. We shortly discuss TeV-scale extensions of the Standard Model
which may lead to a realization of a strongly interacting neutrino scenario. We
emphasize, however, that such a scenario may even be realized within the
standard electroweak model: electroweak instanton/sphaleron induced processes
may get strong at ultrahigh energies. Possible tests of strongly interacting
neutrino scenarios range from observations at cosmic ray facilities and
neutrino telescopes to searches at lepton nucleon scattering experiments.
