  The large statistics collected during the operation of a ZnMoO$_4$ array, for
a total exposure of 1.3 kg $\cdot$ day of $^{100}$Mo, allowed the first
bolometric observation of the two neutrino double beta decay of $^{100}$Mo. The
observed spectrum of each crystal was reconstructed taking into account the
different background contributions due to environmental radioactivity and
internal contamination. The analysis of coincidences between the crystals
allowed the assignment of constraints to the intensity of the different
background sources, resulting in a reconstruction of the measured spectrum down
to an energy of $\sim$300 keV. The half-life extracted from the data is
T$_{1/2}^{2\nu}$= [7.15 $\pm$ 0.37 (stat) $\pm$ 0.66 (syst)] $\cdot$ 10$^{18}$
y.
