  This paper discusses the science case for a sensitive spectro-polarimetric
survey of the microwave sky. Such a survey would provide a tomographic and
dynamic census of the three-dimensional distribution of hot gas, velocity
flows, early metals, dust, and mass distribution in the entire Hubble volume,
exploit CMB temperature and polarisation anisotropies down to fundamental
limits, and track energy injection and absorption into the radiation background
across cosmic times by measuring spectral distortions of the CMB blackbody
emission. In addition to its exceptional capability for cosmology and
fundamental physics, such a survey would provide an unprecedented view of
microwave emissions at sub-arcminute to few-arcminute angular resolution in
hundreds of frequency channels, a data set that would be of immense legacy
value for many branches of astrophysics. We propose that this survey be
carried-out with a large space mission featuring a broad-band polarised imager
and a moderate resolution spectro-imager at the focus of a 3.5m aperture
telescope actively cooled to about 8K, complemented with absolutely-calibrated
Fourier Transform Spectrometer modules observing at degree-scale angular
resolution in the 10-2000 GHz frequency range. We propose two observing modes:
a survey mode to map the entire sky as well as a few selected wide fields, and
an observatory mode for deeper observations of regions of specific interest.
