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Heat flow measurements through old seafloor demonstrate that the oceanic lithosphere is heated from below away from hot spot tracks. We reevaluate the hypothesis of small‐scale convection beneath the lithosphere with laboratory experiments in fluids whose viscosity depends strongly on temperature. Rayleigh numbers were between 10⁶ and 10⁸ and viscosity contrasts were up to 10⁶. A layer of fluid was impulsively cooled from above, and a cold boundary layer grew at the top of the fluid layer. After a finite time, convective instabilities developed in the lowermost part of the boundary layer, while the upper part remained stagnant. The variation of surface heat flow as a function of time reflects the three‐dimensional nature of the flow and the presence of a thick lid. At viscosity contrasts greater than 10³, this variation is very similar to what is observed on the oceanic lithosphere. For small times, heat flow follows the …