  We report the detection of strong absorption and weak emission features in
the 4--27 micron Spitzer-IRS spectrum of the distant ultraluminous infrared
galaxy (ULIRG) IRAS F00183--7111 (z=0.327). The absorption features of CO2 and
CO gas, water ice, hydrocarbons and silicates are indicative of a strongly
obscured (A[9.6]>=5.4; A[V]>=90) and complex line of sight through both hot
diffuse ISM and shielded cold molecular clouds towards the nuclear power
source. From the profile of the 4.67 micron CO fundamental vibration mode we
deduce that the absorbing gas is dense (n~10^6 cm^-3) and warm (720 K) and has
a CO column density of ~10^19.5 cm^-2, equivalent to N[H]~10^23.5 cm^-2. The
high temperature and density, as well as the small infered size (<0.03pc),
locates this absorbing gas close to the power source of this region. Weak
emission features of molecular hydrogen, PAHs and Ne+, likely associated with
star formation, are detected against the 9.7 micron silicate feature,
indicating an origin away from the absorbing region. Based on the 11.2 micron
PAH flux, we estimate the star formation component to be responsible for up to
30% of the IR luminosity of the system. While our mid-infrared spectrum shows
no tell-tale signs of AGN activity, the similarities to the mid-infrared
spectra of deeply obscured sources (e.g. NGC4418) and AGN hot dust (e.g.
NGC1068), as well as evidence from other wavelength regions, suggest that the
power source hiding behind the optically thick dust screen may well be a buried
AGN.
