On Ninetyeast Ridge the seismological community drilled Ocean Drilling Program (ODP) Site 1107, coincident with ODP Site 757, for the future host of a long-term seismological observatory in the Central Indian Ocean. To study the shallow seismic structure at the drill site, we developed a unusual on-bottom seismic tool. Five implosive sources were placed along a 2-km line. Four ocean bottom sensors recorded seismic waves over epicentral distances of 400 m to 2200 m, revealing seismic velocities of the immediately subjacent sediments. Velocities within this portion of crust are generally not detectable by traditional marine seismic techniques. Airgun shots, however, can be used to determine velocities of basement rocks. One-dimensional travel-time inversion of both bottom sources and water path corrected surface shots yields a velocity structure which is in excellent agreement with ODP drilling results. Three lithological units are imaged. A 150 m thick sediment section with velocities of 1600--1720 m/s. Underneath, a layer of 230 m thickness with velocities ranging from 2080 to 2250 m/s, corresponding to volcanic ashes and tuffs. The volcanic basement occured at 380 m below seafloor. Velocities rapidly increase from 3500 m/s to values of more than 4500 m/s within a few hundred metres. ¿ 1999 American Geophysical Union