Petrological and geochemical results from Integrated Ocean Drilling Program (IODP, 2003-2013) analyses of deep-seated mid-ocean ridge rocks (gabbros and peridotites) are summarized. Three important contributions to our understanding of the nature and origin of oceanic core complexes were obtained from: (1) a >1400 m gabbroic section from the Atlantis Massif of the Mid-Atlantic Ridge (site U1309, drilled by Expeditions 304/305), (2) sampling of intact oceanic gabbro after drilling through basaltic and sheeted dike complex layers from the fast-spreading Cocos Plate (leg 206 of site 1256D, drilled by Expeditions 309, 312 and, 335), and (3) sampling of layered gabbro from the Hess Deep of the East Pacific Rise (site U1415, drilled by Expedition 345). Future deep-seated hard-rock drilling projects by the research drilling vessel CHIKYU, including mantle drilling projects, are introduced.