  We have carried out 1.25 pc resolution MHD simulations of the ISM, on a
Cartesian grid of $0 \leq (x,y) \leq 1$ kpc size in the galactic plane and $-10
\leq z \leq 10$ kpc into the halo, thus being able to fully trace the
time-dependent evolution of the galactic fountain. The simulations show that
large scale gas streams emerge, driven by SN explosions, which are responsible
for the formation and destruction of shocked compressed layers. The shocked gas
can have densities as high as 800 cm$^{-3}$ and lifetimes up to 15 Myr. The
cold gas is distributed into filaments which tend to show a preferred
orientation due to the anisotropy of the flow induced by the galactic magnetic
field. Ram pressure dominates the flow in the unstable branch $10^{2}<$T$\leq
10^{3.9}$ K, while for T$\leq 100$ K (stable branch) magnetic pressure takes
over. Near supernovae thermal and ram pressures determine the dynamics of the
flow. Up to 80% of the mass in the disk is concentrated in the thermally
unstable regime $10^{2}<$T$\leq 10^{3.9}$ K with $\sim30%$ of the disk mass
enclosed in the T$\leq 10^{3}$ K gas. The hot gas in contrast is controlled by
the thermal pressure, since magnetic field lines are swept towards the dense
compressed walls.
