  Relying on the dramatic increase in the number of stars with full 6D
phase-space information provided by the Gaia Data Release 3, we discover
unambiguous signatures of phase-mixing in the stellar halo around the Sun. We
show that for the stars likely belonging to the last massive merger, the
(v_r,r) distribution contains a series of long and thin chevron-like
overdensities. These phase-space sub-structures are predicted to emerge
following the dissolution of a satellite, when its tidal debris is given time
to wind up, thin out and fold. Additionally, the observed energy and angular
momentum (E, L_z) distribution appears more prograde at high energies, possibly
revealing the original orbital angular momentum of the in-falling galaxy. The
energy distribution of the debris is strongly asymmetric with a peak at low E
-- which, we surmise, may be evidence of the dwarf's rapid sinking -- and
riddled with wrinkles and bumps. If these small-scale energy inhomogeneities
have been seeded during or immediately after the interaction with the Milky
Way, and are not due to the spatial restriction of our study, then making use
of the (v_r,r) chevrons to constrain the time of the merger becomes cumbersome.
Nonetheless, we demonstrate that similar phase-space and (E,L_z) sub-structures
are present in numerical simulations of galaxy interactions, both in bespoke
N-body runs and in cosmological hydrodynamical zoom-in suites. The remnant
traces of the progenitor's disruption and the signatures of the on-going
phase-mixing discovered here will not only help to constrain the properties of
our Galaxy's most important interaction, but also can be used as a novel tool
to map out the Milky Way's current gravitational potential and its
perturbations.
