  We present the MUSE Hubble Ultra Deep Survey, a mosaic of nine MUSE fields
covering 90\% of the entire HUDF region with a 10-hour deep exposure time, plus
a deeper 31-hour exposure in a single 1.15 arcmin2 field. The improved
observing strategy and advanced data reduction results in datacubes with
sub-arcsecond spatial resolution (0.65 arcsec at 7000 A) and accurate
astrometry (0.07 arcsec rms). We compare the broadband photometric properties
of the datacubes to HST photometry, finding a good agreement in zeropoint up to
mAB=28 but with an increasing scatter for faint objects. We have investigated
the noise properties and developed an empirical way to account for the impact
of the correlation introduced by the 3D drizzle interpolation. The achieved 3
sigma emission line detection limit for a point source is 1.5 and 3.1 10-19
erg.s-1.cm-2 for the single ultra-deep datacube and the mosaic, respectively.
We extracted 6288 sources using an optimal extraction scheme that takes the
published HST source locations as prior. In parallel, we performed a blind
search of emission line galaxies using an original method based on advanced
test statistics and filter matching. The blind search results in 1251 emission
line galaxy candidates in the mosaic and 306 in the ultradeep datacube,
including 72 sources without HST counterparts (mAB>31). In addition 88 sources
missed in the HST catalog but with clear HST counterparts were identified. This
data set is the deepest spectroscopic survey ever performed. In just over 100
hours of integration time, it provides nearly an order of magnitude more
spectroscopic redshifts compared to the data that has been accumulated on the
UDF over the past decade. The depth and high quality of these datacubes enables
new and detailed studies of the physical properties of the galaxy population
and their environments over a large redshift range.
