  The spiral structure of the Milky Way can be simulated by adopting
percolation theory, where the active zones are produced by the evolution of
many supernova (SN). Here we assume conversely that the percolative process is
triggered by superbubbles (SB), the result of multiple SNs. A first thermal
model takes into account a bursting phase which evolves in a medium with
constant density, and a subsequent adiabatic expansion which evolves in a
medium with decreasing density along the galactic height. A second cold model
follows the evolution of an SB in an auto-gravitating medium in the framework
of the momentum conservation in a thin layer. Both the thermal and cold models
are compared with the results of numerical hydro-dynamics. A simulation of
GW~46.4+5.5, the Gould Belt, and the Galactic Plane is reported. An elementary
theory of the image, which allows reproducing the hole visible at the center of
the observed SB, is provided.
