  The study of transiently accreting neutron stars provides a powerful means to
elucidate the properties of neutron star crusts. We present extensive numerical
simulations of the evolution of the neutron star in the transient low-mass
X-ray binary MAXI J0556--332. We model nearly twenty observations obtained
during the quiescence phases after four different outbursts of the source in
the past decade, considering the heating of the star during accretion by the
deep crustal heating mechanism complemented by some shallow heating source. We
show that cooling data are consistent with a single source of shallow heating
acting during the last three outbursts, while a very different and powerful
energy source is required to explain the extremely high effective temperature
of the neutron star, ~350 eV, when it exited the first observed outburst. We
propose that a gigantic thermonuclear explosion, a "hyperburst" from unstable
burning of neutron rich isotopes of oxygen or neon, occurred a few weeks before
the end of the first outburst, releasing 10^44 ergs at densities of the order
of 10^11 g/cm^3. This would be the first observation of a hyperburst and these
would be extremely rare events as the build up of the exploding layer requires
about a millennium of accretion history. Despite its large energy output, the
hyperburst did not produce, due to its depth, any noticeable increase in
luminosity during the accretion phase and is only identifiable by its imprint
on the later cooling of the neutron star.
