In October 1978, the U.S. Geological Survey established a 100-km-long seismic refraction profile normal to the Kona coast of Hawaii Island. Analysis of these data along with available gravity data suggest that the oceanic crust dips about 3° landward under the submerged flank of the island increasing to 8.5° under the Kona coast. Maximum vertical deflection of the base of crust from beneath the deep ocean to a point beneath the summit of Mauna Loa is about 9 km. High-velocity, high-density rocks (Vp about 7.1 km/sec, density about 2.9 gm/cm3) comprise the bulk of the volcanic edifice and reach to within a few kilometers of the surface beneath the north flank of Mauna Loa. The data also suggest that an elongate high-velocity, high-density body lies parallel to the Kona coast just below the surface; this body probably represents an extinct, buried rift zone.