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 is hidden from direct observations. Recent studies suggest the "internal-external" model: internal shocks that take place within the relativisticjlow produce the GRB while the subsequent interaction of the j-low 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.