Ocean Drilling Program Leg 191 consists of two parts: (1) a science segment devoted to drilling and casing a hole on the northwest Pacific abyssal seafloor (Site WP-2) coupled with the installation of a broadband seismometer for a long-term sub-seafloor borehole observatory and (2) engineering tests of the hard rock reentry system (HRRS) and other equipment. The seismic observatory is an important component of the International Ocean Network seismometer net. By filling a large gap in the global station grid, it will help increase the resolution of global tomographic studies, which have revolutionized understanding of mantle dynamics and structure. Moreover, it will allow more precise studies of the seismic structure of old Pacific Ocean crust and lithosphere, as well as better resolution of earthquake locations and mechanisms in the northwest Pacific subduction zone. Approximately 400 m of sediments and 100 m of basalt will be cored at Site WP-2. Studies of these cores will add to existing knowledge of Cretaceous Pacific mid-ocean ridge basalt chemistry, construction of the ocean crust, paleolatitude, the age of magnetic lineations, basalt physical properties, and the deep biosphere. Engineering tests will be conducted to test the performance of hammers and bits for the HRRS that will eventually allow hard-rock spudding of holes at mid-ocean ridges and other locales where holes must be started on hard outcrops. The hammer tests are proposed for Site SR-1, a bare basalt outcrop atop Shatsky Rise. Engineering goals may also include testing of the Hydrate Autoclave Coring Equipment System.