There is an excellent correlation between tomographic patterns and global tectonics in the upper 200 km of the mantle. Below this depth there is little relationship with present tectonic provinces. Most of the power in the tomography is in the longest wavelengths (l=1--3) with l=2 generally dominant. This is quite different from expectations based on mantle convection models. The question then arises, what causes the long wavelength variations mapped by seismic techniques? We investigate the hypotheses that past locations of supercontinents and subduction may control temperature variations in the mantle. We look for correlations between Pangea and post-Pangeatic subduction and seismic velocity variations at various depths. The best correlations with time integrated slab locations occur near the base of the uppermantle, suggesting that slabs bottom out in the mesosphere. The Pangea hemisphere has a colder than average uppermantle, probably due to circum- and intra-Pangea subduction. Hotspots and ridges avoid regions cooled by subduction over the past 180 Ma. The presence of large continents at the surface and large areas of cold slab at depth explain the dominance of long wavelengths in the mantle's thermal structure, despite the high Rayleigh number. ¿ American Geophysical Union 1992