Seamounts are not solid basalt structures, but have relatively high porosities in their upper crustal sections. At Axial Seamount, off the coast of Oregon, United States, we used on-bottom gravity measurements with a Bell gravity meter within deep-sea submersible Alvin to determine a porosity of 31% for the uppermost ∼100 m of the edifice. The southwestern caldera wall has a porosity of 22% and the caldera floor has a slightly higher porosity of 33%. Seafloor observations and models indicate that these high porosities result from large-scale structural features such as lava tubes and cracks, large lava drain backs, or regions of open pillow basalts in the near subsurface. These high-porosity zones can affect the subsurface permeability, and models of hydrothermal upflow zones explain observed localized gravity anomalies. The variety of hydrothermal alteration, hydrothermally active areas, and open porous features appears to be related to the high porosity that is inferred from geophysical measurements on this active seafloor volcano.