A wide diversity of astrophysical phenomena - from stellar flares in the solar neighborhood to accretion in galactic nuclei at cosmological distances - are characterized by flux and spectral changes on time scales ranging from a fraction of a second to several years. Every day, observing facilities with time-resolved imaging capabilities collect huge amounts of potentially interesting information, which remains mostly unused, stored in data archives. This is especially true in the high energy range of the electromagnetic spectrum, where source variability is very common but the time dimension is seldom systematically exploited.Treasuring on the experience of our European team, we propose an unprecedented program to search and characterize variable sources in the soft X-ray energy range. To do so we’ll exploit the whole database collected by the EPIC cameras onboard the XMM-Newton mission. This will include a search for fast transients, missed by standard image analysis, as well as a search and characterization of variability in tens of thousands of sources on a broad range of time scales.X-ray results will be complemented by multiwavelength characterization of all previously undetected sources. Phenomenological classification of variable sources will also be performed. Our project will certainly unveil new and unexpected classes of sources (as has always been the case when a new region in parameter space has been explored) enhancing the potential of discovery of the XMM-Newton mission, in itself the most productive observatory of the European Space Agency. All our results will be made publicly available, together with new analysis tools. Our variable source catalogue will trigger reanalysis of other databases as well as new observations and will become a reference in the forecoming era of large surveys. As the most sensitive search for variability ever performed, it may raise new questions in high-energy astrophysics and serve as a pathfinder for future missions.