The above shows the result of topography measurements for the 660 km discontinuity; the average of all 470 cap values is 654 km. Each symbol represents the measured depth perturbation from the average at the corresponding cap location. Using the least squares method with damping, we fit the values of spherical harmonics up to degree 12 (169 coefficients). The color contours show the values obtained by the synthesis of the spherical harmonic expansion. The most notable features are the large depression of the discontinuity boundary in the western Pacific, Tonga and South America. With the most dense data coverage included in this study, the depression under the western Pacific extending from Kamchatka to the Philippines is the most robust of the long wavelength features. The depth perturbations are as large as 25 km from the global mean; this result is consistent with earlier works, and its cause has been associated with the effect of subduction. No significant features are evident near the East Pacific Rise.