Publication
Context. Spectroscopic surveys of massive galaxy clusters reveal the properties of faint background galaxies thanks to the magnification provided by strong gravitational lensing.Aims. We present a systematic analysis of integral-field-spectroscopy observations of 12 massive clusters, conducted with the Multi Unit Spectroscopic Explorer (MUSE). All data were taken under very good seeing conditions (similar to 0 ''.6) in effective exposure times between two and 15 h per pointing, for a total of 125 h. Our observations cover a total solid angle of similar to 23 arcmin(2) in the direction of clusters, many of which were previously studied by the MAssive Clusters Survey, Frontier Fields (FFs), Grism Lens-Amplified Survey from Space and Cluster Lensing And Supernova survey with Hubble programmes. The achieved emission line detection limit at 5 sigma for a point source varies between (0.77-1.5) x 10(-18) erg s(-1) cm(-2) at 7000 angstrom.Methods. We present our developed strategy to reduce these observational data, detect continuum sources and line emitters in the datacubes, and determine their redshifts. We constructed robust mass models for each cluster to further confirm our redshift measurements using strong-lensing constraints, and identified a total of 312 strongly lensed sources producing 939 multiple images.Results. The final redshift catalogues contain more than 3300 robust redshifts, of which 40% are for cluster members and similar to 30% are for lensed Lyman-alpha emitters. Fourteen percent of all sources are line emitters that are not seen in the available HST images, even at the depth of the FFs (similar to 29 AB). We find that the magnification distribution of the lensed sources in the high-magnification regime (mu=2-25) follows the theoretical expectation of N(z) proportional to mu(-2). The quality of this dataset, number of lensed sources, and number of strong-lensing constraints enables detailed studies of the physical properties of both the lensing cluster and the background galaxies. The full data products from this work, including the datacubes, catalogues, extracted spectra, ancillary images, and mass models, are made available to the community.