  First order phase transitions are violent phenomena that occur when the state
of the universe evolves abruptly from one vacuum to another. A \emph{direct}
phase transition connects a local vacuum to a deeper vacuum of the
zero--temperature potential, and the energy difference between the two minima
manifests itself in the acceleration of the bubble wall. In this sense, the
transition is triggered by the release of vacuum energy. On the other hand, an
\emph{inverse} phase transition connects a deeper minimum of the
zero--temperature potential to a higher one, and the bubble actually expands
against the vacuum energy. The transition is then triggered purely by thermal
corrections. We study for the first time the hydrodynamics and the energy
budget of inverse phase transitions. We find several modes of expansion for
inverse bubbles, which are related to the known ones for direct transitions by
a mirror symmetry. We finally investigate the friction exerted on the bubble
wall and comment on the possibility of runaway walls in inverse phase
transitions.
