The exposure of 1300 meters of upper oceanic crust at the Blanco Fracture Zone allows near-bottom gravity measurements to determine the in situ density of the seafloor as a function of depth. Gravity measurements along the north wall of the Blanco Depression indicate an outcrop density of 2530¿Kg/m3 for the upper 800 meters of crust and a calculated porosity of 23%. The lower 500 meters of crust (800 to 1300 meters below the sea floor) has a measured density of 2710¿130 Kg/m3 and a porosity of 14%. These data indicate that most of the extrusive volcanic oceanic crust is highly porous and can act as an aquifer and large-scale reservoir for hydrothermal fluids. These direct crustal density measurements also support previous interpretations that low seismic velocities observed in Layer 2 are due to the high porosity of the upper extrusive section. ¿ 2000 American Geophysical Union