  Classical MHD reconnection theories, both the stationary Sweet-Parker model
and the tearing instability, are known to provide rates which are too slow to
explain the observations. However, a recent analysis has shown that there
exists a critical threshold on current sheet's thickness, namely a/L~S^(-1/3),
beyond which the tearing modes evolve on fast macroscopic Alfvenic timescales,
provided the Lunquist number S is high enough, as invariably found in solar and
astrophysical plasmas. Therefore, the classical Sweet-Parker scenario, for
which the diffusive region scales as a/L~S^(-1/2) and thus can be up to ~100
times thinner than the critical value, is likely to be never realized in
nature, as the current sheet itself disrupts in the elongation process. We
present here two-dimensional, compressible, resistive MHD simulations, with S
ranging from 10^5 to 10^7, that fully confirm the linear analysis. Moreover, we
show that a secondary plasmoid instability always occurs when the same critical
scaling is reached on the local, smaller scale, leading to a cascading
explosive process, reminiscent of the flaring activity.
