  The discovery of ultrastable glasses has raised novel challenges about glassy
systems. Recent experiments studied the macroscopic devitrification of
ultrastable glasses into liquids upon heating but lacked microscopic
resolution. We use molecular dynamics simulations to analyse the kinetics of
this transformation. In the most stable systems, devitrification occurs after a
very large time, but the liquid emerges in two steps. At short times, we
observe the rare nucleation and slow growth of isolated droplets containing a
liquid maintained under pressure by the rigidity of the surrounding glass. At
large times, pressure is released after the droplets coalesce into large
domains, which accelerates devitrification. This two-step process produces
pronounced deviations from the classical Avrami kinetics and explains the
emergence of a giant lengthscale characterising the devitrification of bulk
ultrastable glasses. Our study elucidates the nonequilibrium kinetics of
glasses following a large temperature jump, which differs from both equilibrium
relaxation and aging dynamics, and will guide future experimental studies.
