  The microscopic mechanism of the melting of a crystal is analyzed by the
constant pressure Monte Carlo simulation of a Lennard-Jones fcc system. Beyond
a temperature of the order of 0.8 of the melting temperature, we found that the
relevant excitations are lines of defects. Each of these lines has the
structure of a random walk of various lengths on an fcc defect lattice. We
identify these lines with the dislocation ones proposed in recent
phenomenological theories of melting. Near melting we find the appearance of
long lines that cross the whole system. We suggest that these long lines are
the precursor of the melting process.
