  Supercritical fluids play a significant role in elucidating fundamental
aspects of liquid matter under extreme conditions. They have been extensively
studied at pressures and temperatures relevant to various industrial
applications. However, much less is known about the structural behaviour of
supercritical fluids and no structural crossovers have been observed in static
compression experiments in any temperature and pressure ranges beyond the
critical point. The structure of supercritical state is currently perceived to
be uniform everywhere on the pressure-temperature phase diagram, and to change
only in a monotonic way even moving around the critical point, not only along
isotherms or isobars. Conversely, we observe structural crossovers for the
first time in a deeply supercritical sample through diffraction measurements in
a diamond anvil cell and discover a new thermodynamic boundary on the
pressure-temperature diagram. We explain the existence of these crossovers in
the framework of the phonon theory of liquids using molecular dynamics
simulations. The obtained results are of prime importance since it implies a
global reconsideration of the mere essence of the supercritical phase.
Furthermore, this discovery may pave the way to new unexpected applications and
to the exploration of exotic behaviour of confined fluids relevant to geo- and
planetary sciences.
