  We report the observation of superstructures, i.e.\ very large-scale and long
living coherent structures in highly turbulent Rayleigh-B\'enard convection up
to Rayleigh $Ra=10^9$. We perform direct numerical simulations in horizontally
periodic domains with aspect ratios up to $\Gamma=128$. In the considered $Ra$
number regime the thermal superstructures have a horizontal extend of six to
seven times the height of the domain and their size is independent of $Ra$.
Many laboratory experiments and numerical simulations have focused on small
aspect ratio cells in order to achieve the highest possible $Ra$. However, here
we show that for very high $Ra$ integral quantities such as the Nusselt number
and volume averaged Reynolds number only converge to the large aspect ratio
limit around $\Gamma \approx 4$, while horizontally averaged statistics such as
standard deviation and kurtosis converge around $\Gamma \approx 8$, and the
integral scale converges around $\Gamma \approx 32$, and the peak position of
the temperature variance and turbulent kinetic energy spectra only around
$\Gamma \approx 64$.
