The interpretation of the seismic D\ layer at the base of the mantle as the lower thermal boundary layer of a whole mantle convective circulation has several important geophysical consequences. Because of the strong temperature dependence of mantle viscosity, which sharply reduces the viscosity in the boundary layer itself, this region could be strongly unstable against secondary convective instability. Linear stability analyses presented here show that this destabilization is in fact dramatic, the fastest growing disturbances being characterized by growth times on the order of 106 years and spatial scales on the order of 102 km for a wide variety of plausible lower mantle conditions. The highly nonlinear evolution of the resulting ''thermal turbulence'' may be such that fastest growing disturbances mature into thermal plumes which rise rapidly to the base of the lithosphere where they may induce intense partial melting.