Core–mantle boundary Core–mantle boundary The core–mantle boundary (CMB in the parlance of solid earth geophysicists) of the Earth lies between the planet's silicate mantle and its liquid iron-nickel outer core. This boundary is located at approximately 2891 km (1796 mi) depth beneath the Earth's surface. The boundary is observed via the discontinuity in seismic wave velocities at that depth due to the differences between the acoustic impedances of the solid mantle and the molten outer core. P-wave velocities are much slower in the outer core than in the deep mantle while S-waves do not exist at all in the liquid portion of
What Shall We Do Now? sped up, to raise its key to E minor, with re-recorded vocals and guitar. The members of Pink Floyd have contributed to the confusion regarding the identity of this track, misidentifying "What Shall We Do Now?" as "Empty Spaces" on multiple occasions, such as in the track listing for the film version of "The Wall", and on Waters' "The Wall Live in Berlin". On other occasions (such as the officially released live version), the first and second parts of the track are divided and identified as "Empty Spaces" and "What Shall We Do Now?" respectively, even though they are in