  Gamma-ray bursts (GRBs) have puzzled astronomers since their accidental
discovery in the sixties. The BATSE detector on the COMPTON-GRO satellite has
been detecting one burst per day for the last six years. Its findings have
revolutionized our ideas about the nature of these objects. They have shown
that GRBs are at cosmological distances. This idea was accepted with
difficulties at first. However, the recent discovery of an x-ray afterglow by
the Italian/Dutch satellite BeppoSAX led to a detection of high red-shift
absorption lines in the optical afterglow of GRB970508 and to a confirmation of
its cosmological origin. The simplest and practically inevitable interpretation
of these observations is that GRBs result from the conversion of the kinetic
energy of ultra-relativistic particles flux to radiation in an optically thin
region. The "inner engine" that accelerates the particles or generates the
Poynting flux is hidden from direct observations. Recent studies suggest the
``internal-external'' model: internal shocks that take place within the
relativistic flow produce the GRB while the subsequent interaction of the flow
with the external medium produce the afterglow. The ``inner engine'' that
produces the flow is, however, hidden from direct observations. We review this
model with a specific emphasis on its implications to underground physics.
