Deeply x-raying the high-energy sky

All-sky explorations by Fermi-LAT have revolutionized our view of the gamma-ray sky. While its ongoing all-sky survey counts thousands of sources, essential issues related to the nature of unassociated sources call for sensitive all-sky surveys at hard X-ray energies that allow for their identification. We present the results of the association of the Fermi-LAT second source catalog to hard X-ray detected sources.