During the last decade, Integrated Ocean Drilling Program (IODP) has made substantial progress in sampling the deeper parts of the ocean crust (>1 km below seafloor). Holes into slow-spread Atlantic ocean crust at Site U1309 and fast-spread Pacific crust at Site 1256 shed light on important unknown and poorly understood aspects of the architecture and accretion of modern ocean crust. Recent drilling into Pacific lower crust tectonically exposed at Hess Deep also provides new insights (Site U1415). Key observations and findings include the possible implication of mantle rocks in magmatic accretion of the lower crust at slow-spreading ridges, the confirmation that the development of oceanic core complexes via detachment faulting involves tectonic rotation of the footwall, a detailed and comprehensive picture of the complex interplay between magmatic, metamorphic, and hydrothermal processes at the base of the sheeted dike complex at fast-spreading ridges, and the first sampling of significant sections of layered gabbros in fast-spread lower crust.