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In 1997, a Rayleigh-B\'enard experiment evidenced a significant increase ofthe heat transport efficiency for Rayleigh numbers larger than $Ra \sim10^{12}$ and interpreted this observation as the signature of the Kraichnan's``Ultime Regime'' of convection. According to Kraichnan's 1962 prediction, theflow boundary layers above the cold and hot plates -in which most of the fluidtemperature drop is localized- become unstable for large enough $Ra$ and thisinstability boosts the heat transport compared to the other turbulent regimes.Using the same convection cell as in the 1997 experiment, we show that thereported heat transport increase is accompanied with enhanced temperaturefluctuations of the bottom plate, which was heated at constant power levels.Indeed, for $Ra < 10^{12}$, the bottom plate fluctuations can simply beaccounted from those in the bulk of the flow. In particular, they share thesame spectral density at low frequencies, as if the bottom plate was followingthe slow temperature fluctuations of the bulk, modulo a constant temperaturedrop across the bottom boundary layer. Conversely, to account for the plate'stemperature fluctuations at higher $Ra$, we no-longuer can ignore thefluctuations of the temperature drop across the boundary layer. The negativeskewness of fluctuations at high $Ra$ supports the picture of a boundary layerinstability. These observations provide new evidence that the transitionreported in 1997 corresponds to the triggering of the Ultimate Regime ofconvection.

Evidence of a boundary layer instability at very high Rayleigh number