The composition and elastic properties of the earth's lithosphere benheath the sea floor are fundamental to our understanding of the mechanism of sea floor spreading and continental drift. New experimental information on the elastic properties, as a function of temperature, pressure, and petrology, of eclogite and peridotite is presented. The density and seismic wave velocities in peridotite simulated in the laboratory for the oceanic lithosphere of the first 15--20 km of depth match results of of recent seismic investigations very well. The elastic properties of olivine eclogite describe the seismic structure of the remaining lithosphere. The present study favors the idea of a chemical change within the lithosphere, and our laboratory results tend to favor ?~22 for the oceanic lithosphere. In modeling the structure in terms of temperature and pressure coefficients the surface wave studies introduce two complications: First, since partial melting is required by the very low shear wave velocities of the asthenosphere, some consistent and sensible way of treating velocities in mush must be used. Second, the mantle is anisotropic; therefore a systematic crystal orientation has to be considered rather than just the average Vp and Vs for the mantle constituents. The seismic anomalies of azimuth-dependent fluctuations in the velocity of Pn waves along the base of the oceanic crust are as much as 8%; the high velocity tends to be in the direction of sea floor spreading. A consistent explanation of this effect would be the presence of a sustained extensional strain rate in the spreading direction, applied at the base of the oceanic lithosphere. It would appear that the anisotropy vanishes near the surface of the oceanic plate.