The dense hot intracluster gas in the central 100-200 kpc of most
 clusters of galaxies is cooling on a timescale of less than 10
 billion yrs leading to cooling flows at rates of 10s to 100s Msun
 /yr. This must cause the accumulation of 1e11 to 1e13 Msun of
 cooled material around the central cluster galaxies. Absorption
 seen in X-ray spectra indicates that much of this cooled matter
 remains as cold gas, probably in the form of very cold clouds.
 Part of the cooled product of flows may form stars, which must
 be of low mass in order that optical magnitudes are not exceeded.
 We propose here to search for line radiation from the cold gas
 clouds at [OI]63mu and [SiII]34.8mu which is predicted to
 emerge from throughout the cooling flow. The discovery of
 such extended infrared radiation would open up this new component
 of the intracluster medium for detailed study. It has widespread
 implications for the formation of the most massive galaxies and
 for baryonic dark matter, as well as the nature of the whole
 intracluster medium, which may be multiphase.
 The numbers of clouds in the inner few kpc of a massive flow are
 so high that cloud-cloud collisions and aggregation occur, leading
 to optical nebulosities and massive star formation in some
 central cluster galaxies. We further propose to observe this
 region in such galaxies, where dust, possibly formed in the dense
 clouds, may obscure much of the action in the optical band.
 The objects proposed for here are the most massive, nearby cooling
 flows and are the most massive and extensive regions of cold gas at
 low redshifts. ISO can provide vital direct information on the
 nature and evolution of this newly-recognized major component of
 the Universe.