  We report new single dish CO J=6--5 line observations for the archetypal
Ultra Luminous Infrared Galaxy (ULIRG) Arp 220 with the James Clerk Maxwell
Telescope atop Mauna Kea in Hawaii. The J=6--5 line is found to be faint, with
brightness temperature ratios (6--5)/(1--0), (6--5)/(3--2) of R_{65/10} =
0.080+/-0.017 and R_{65/32} = 0.082+/-0.019, suggesting very low excitation
conditions that cannot be reconciled with the warm and very dense molecular gas
present in one of the most extreme starbursts in the local Universe. We find
that an optically thick dust continuum, with tau(f>350 GHz)>=1 for the bulk of
the warm dust and gas in Arp 220, submerges this line to an almost black body
curve, reducing its flux, and {\it affecting its CO Spectral Line Energy
Distribution (SLED) at high frequencies}. This also resolves the CII line
deficiency in this object, first observed by {\it ISO}: the near absence of
that line is a dust optical depth effect, not a dense Photodissociation Region
(PDR) phenomenon. Finally we briefly comment on the possibility of such extreme
ISM states in other ULIRGs in the distant Universe, and its consequences for
the diagnostic utility of high frequency molecular and atomic ISM lines in such
systems. In the case of Arp 220 we anticipate that the now spaceborne Herschel
Space Observatory will find faint high-J CO lines at f>=690 GHz that would
appear as sub-thermally excited with respect to the low-J ones as a result of
the effects of dust absorption.
