Cosmic rays hit the nucleons in the atmosphere
with energies of up to 10^12 GeV. The extensive air showers
that they produce are then a good place to look for signals
of new physics. In particular, ultraenergetic cosmic rays could
probe the models with a low energy scale of unification with
gravity. I will discuss the neutrino-nucleon cross section in
a string model with the fundamental scale at the TeV.