  We report angle-resolved photoemission spectroscopy (ARPES) experiments
probing deep into the hidden order (HO) state of URu2Si2, utilizing tunable
photon energies with sufficient energy and momentum resolution to detect the
near Fermi surface (FS) behavior. Our results reveal: (i) the full itinerancy
of the 5f electrons; (ii) the crucial three-dimensional (3D) k-space nature of
the FS and its critical nesting vectors, in good comparison with
density-functional theory calculations, and (iii) the existence of hot-spot
lines and pairing of states at the FS, leading to FS gapping in the HO phase.
