  In scenarios with extra dimensions and TeV-scale quantum gravity, black holes
are expected to be produced copiously at center-of-mass energies above the
fundamental Planck scale. The Large Hadron Collider (LHC) may thus turn into a
factory of black holes, at which their production and evaporation may be
studied in detail. But even before the LHC starts operating, the Pierre Auger
Observatory for cosmic rays, presently under construction, has an opportunity
to search for black hole signatures. Black hole production in the scattering of
ultrahigh energy cosmic neutrinos on nucleons in the atmosphere may initiate
quasi-horizontal air showers far above the Standard Model rate. In this letter,
we compare the sensitivity of LHC and Auger to black hole production by
studying their respective reach in black hole production parameter space.
Moreover, we present constraints in this parameter space from the
non-observation of horizontal showers by the Fly's Eye collaboration. We find
that if the ultrahigh energy neutrino flux is at the level expected from cosmic
ray interactions with the cosmic microwave background radiation, Auger has only
a small window of opportunity to detect black holes before the start of the
LHC. If, on the other hand, larger ultrahigh energy neutrino fluxes on the
level of the upper limit from ``hidden'' hadronic astrophysical sources are
realized in nature, then the first signs of black hole production may be
observed at Auger. Moreover, in this case, the Fly's Eye constraints, although
more model dependent, turn out to be competitive with other currently available
constraints on TeV-scale gravity which are mainly based on interactions
associated with Kaluza-Klein gravitons.

High Energy Physics - Phenomenology;Astrophysics;General Relativity and Quantum Cosmology;High Energy Physics - Experiment;High Energy Physics - Theory