  We report the discovery of an intriguing pressure-driven phase transformation
in the layered Kitaev-material $\alpha$-RuCl$_3$. By analyzing both the Bragg
scattering as well as the diffuse scattering of high-quality single crystals,
we reveal a collective reorganization of the layer stacking throughout the
crystal. Importantly, this transformation also effects the structure of the
RuCl$_3$ honeycomb layers, which acquire a high trigonal symmetry with a single
Ru--Ru distance of 3.41\r{A} and a single Ru--Cl--Ru bond angle of 92.8{\deg}.
Hydrostatic pressure therefore allows to tune the structure of
$\alpha$-RuCl$_3$ much closer to the ideal Kitaev-limit. The high-symmetry
phase can also be stabilized by biaxial stress, which can explain conflicting
results reported earlier and, more importantly, makes the high-symmetry phase
accessible to a variety of experiments.
