  The evolution of the Milky Way disk, which contains most of the stars in the
Galaxy, is affected by several phenomena. For example, the bar and the spiral
arms of the Milky Way induce radial migration of stars and can trap or scatter
stars close to orbital resonances. External perturbations from satellite
galaxies can also have a role, causing dynamical heating of the Galaxy,
ring-like structures in the disk and correlations between different components
of the stellar velocity. These perturbations can also cause 'phase wrapping'
signatures in the disk, such as arched velocity structures in the motions of
stars in the Galactic plane. Some manifestations of these dynamical processes
have already been detected, including kinematic substructure in samples of
nearby stars, density asymmetries and velocities across the Galactic disk that
differ from the axisymmetric and equilibrium expectations, especially in the
vertical direction, and signatures of incomplete phase mixing in the disk. Here
we report an analysis of the motions of six million stars in the Milky Way
disk. We show that the phase-space distribution contains different
substructures with various morphologies, such as snail shells and ridges, when
spatial and velocity coordinates are combined. We infer that the disk must have
been perturbed between 300 million and 900 million years ago, consistent with
estimates of the previous pericentric passage of the Sagittarius dwarf galaxy.
Our findings show that the Galactic disk is dynamically young and that
modelling it as time-independent and axisymmetric is incorrect.
