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