  We observed the Hubble Deep Field South with the new panoramic integral field
spectrograph MUSE that we built and just commissioned at the VLT. The data cube
resulting from 27 hours of integration covers one arcmin^2 field of view at an
unprecedented depth with a 1 sigma emission line surface brightness limit of
1x$10^{-19}$ erg/s/cm$^2$/arcsec$^2$ and contains ~90,000 spectra. We present
the combined and calibrated data cube, and we perform a first-pass analysis of
the sources detected in the HDF-S imaging. We measured the redshifts of 189
sources up to a magnitude F814W = 29.5, increasing by more than an order of
magnitude the number of known spectroscopic redshifts in this field. We also
discovered 26 Lya emitting galaxies which are not detected in the HST WFPC2
deep broad band images.
  The intermediate spectral resolution of 2.3{\AA} allows us to separate
resolved asymmetric Lya emitters, [O II] emitters, and C III] emitters and the
large instantaneous wavelength range of 4500{\AA} helps to identify single
emission lines. We also show how the three dimensional information of MUSE
helps to resolve sources which are confused at ground-based image quality.
  Overall, secure identifications are provided for 83% of the 227 emission line
sources detected in the MUSE data cube and for 32% of the 586 sources
identified in the HST catalog of Casertano et al 2000. The overall redshift
distribution is fairly flat to z=6.3, with a reduction between z=1.5 to 2.9, in
the well-known redshift desert. The field of view of MUSE also allowed us to
detect 17 groups within the field. We checked that the number counts of [O II]
and Ly-a emitters are roughly consistent with predictions from the literature.
Using two examples we demonstrate that MUSE is able to provide exquisite
spatially resolved spectroscopic information on intermediate redshift galaxies
present in the field.
