  We present results from the combination of two Chandra pointings of the
central region of the cluster of galaxies A3667. From the data analysis of the
first pointing Vikhlinin et al. reported the discovery of a prominent cold
front which is interpreted as the boundary of a cool gas cloud moving through
the hotter ambient gas. Vikhlinin et al. discussed the role of the magnetic
fields in maintaining the apparent dynamical stability of the cold front over a
wide sector at the forward edge of the moving cloud and suppressing transport
processes across the front. In this Letter, we identify two new features in the
X-ray image of A3667: i) a 300 kpc arc-like filamentary X-ray excess extending
from the cold gas cloud border into the hotter ambient gas; ii) a similar
arc-like filamentary X-ray depression that develops inside the gas cloud. The
temperature map suggests that the temperature of the filamentary excess is
consistent with that inside the gas cloud while the temperature of the
depression is consistent with that of the ambient gas. We suggest that the
observed features represent the first evidence for the development of a large
scale hydrodynamic instability in the cluster atmosphere resulting from a major
merger. This result confirms previous claims for the presence of a moving cold
gas cloud into the hotter ambient gas. Moreover it shows that, although the gas
mixing is suppressed at the leading edge of the subcluster due to its magnetic
structure, strong turbulent mixing occurs at larger angles to the direction of
motion. We show that this mixing process may favor the deposition of a
nonnegligible quantity of thermal energy right in the cluster center, affecting
the development of the central cooling flow.
