The geophysical arguments relevant to the depth extent of mantle convection are reviewed with special attention to the seismic properties of subducted lithosphere. New focal mechanism solutions of deep and intermediate earthquakes from the Tonga-Kermadec region are presented, but it is then shown that these do not provide an unambiguous constraint regarding the ability of subducted material to penetrate below 700 km. Estimates of the seismic energy release as a function of depth in Tonga show very large energy release in the depth range 500--700 km, which is just above the depth at which all seismicity ends. This property of the seismicity proves to be the most suggestive of the inability of subducted meterial to penetrate below 700-km depth.