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 \pm 0.017 and R _ { 65 / 32 } =0.082 \pm 0.019 , suggesting very low excitation conditions that can not 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 ( \nu \gtrsim 350 GHz ) \gtrsim 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 affecting its CO Spectral Line Energy Distribution ( SLED ) at high frequencies . This also resolves the C ^ { + } line deficiency in this object , first observed by 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 \nu \gtrsim 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 .