of the so-called neutrino anomalies, a set of circumstantial evidences of electron neutrino disappearance observed at LSND, MiniBooNE, with nuclear reactors and with solar neutrino Gallium detectors (GALLEX/GNO, SAGE). If successful, SOX would demonstrate the existence of sterile neutrino components and open a brand new era in fundamental particle physics and cosmology. A solid signal would mean the discovery of the first particles beyond the Standard Electroweak Model and would have profound implications in our understanding of the Universe and of fundamental particle physics. In case of a negative result, it would be able to close a long-standing debate about the reality of the neutrino anomalies, would probe the existence of new physics in low energy neutrino interactions, would provide a measurement of neutrino magnetic moment, Weinberg angle and other basic physical parameters; and would yield a superb energy calibration for Borexino which will be very beneficial for future high-precision solar neutrino measurements. SOX was envisioned to use a powerful (≈150 kCi) and innovative antineutrino generator made of Ce-144/Pr-144 and possibly a later Cr-51 neutrino generator, which would require a much shorter data-taking campaign. These generators would be located at short distance (8.5 m) from the Borexino detector -under it, in fact: in a pit built ex-profeso before the detector was erected, with the idea it could be used for the insertion of such radioactive sources- and would yield tens of thousands of clean neutrino interactions in the internal volume of the Borexino detector. A high precision (<1% uncertainty) twin-calorimetry campaign would be carried out before deployment in the pit, at the end of data-taking and possibly at some point during the experimental run, in order to provide an independent precise measurement of the source's activity, in order to accomplish a low-uncertainty rate analysis. Shape analyses for the source's antineutrino signal have also been developed in order to increase the experiment's sensitivity, covering the whole high-significance "anomaly" phase space that is still left where light sterile neutrinos could lie in. SOX cancelled The experiment was expected to start in the first half of 2018 and take data for about two years. In October 2017, an end-to-end "blank" (without radioactive material) transport test was carried out successfully at the Borexino site in LNGS, in order to clear out final regulatory permissions for the start of the experiment, ahead of the arrival of the source. The cerium oxide (ceria, or CeO2) source