A series of 10 articles to be published in the journal “Astronomy & Astrophysics” describes various aspects of the deepest spectroscopic investigation ever carried out with the MUSE instrument installed on ESO’s Very Large Telescope (VLT) in Chile. An international team with many researchers focused on the Hubble Space Telescope’s Ultra Deep Field (HUDF) to measure distances and properties of 1,600 very dim galaxies, of which 72 were never detected before.