  Massive stars blow powerful winds and eventually explode as supernovae. By
doing so, they inject energy and momentum in the circumstellar medium, which is
pushed away from the star and piles up to form a dense and expanding shell of
gas. The effect is larger when many massive stars are grouped together in bound
clusters or associations. Large cavities form around clusters as a result of
the stellar feedback on the ambient medium. They are called superbubbles and
are characterised by the presence of turbulent and supersonic gas motions. This
makes star clusters ideal environments for particle acceleration, and potential
contributors to the observed Galactic cosmic ray intensity. The acceleration of
particles at star clusters and in their surroundings may provide a major
contribution to the observed CR flux. Moreover, it may explain the fine
structures observed in the chemical composition of these particles, and
possibly provide a solution to the puzzle of the origin of cosmic rays of
energies in the PeV range and beyond.
