The ISM in the Galactic disk is dominated by Population I stars that fill the
disk. 40% of these stars are found isolated and randomly distributed in the
disk, whereas the remaining 60% of stars are clustered in OB associations.
As these stars becom
e supernovae, they generate high velocity shocks that expand in the surrounding
medium, engulfing it in their passage, thereby shocking, compressing and
heating it. A three-dimensional model for the evolution of the disk gas is
presented here. This model
reproduces many of the features that have been observed in the Galaxy, by means
of 21 cm line emission surveys. The cold structures obtained in the model are
associated with supernova shells, chimney walls and with the growth of
Rayleigh-Taylor instabilities at the base of the Lockman layer.