  The SILCC project (SImulating the Life-Cycle of molecular Clouds) aims at a
more self-consistent understanding of the interstellar medium (ISM) on small
scales and its link to galaxy evolution. We present three-dimensional
(magneto)hydrodynamic simulations of the ISM in a vertically stratified box
including self-gravity, an external potential due to the stellar component of
the galactic disc, and stellar feedback in the form of an interstellar
radiation field and supernovae (SNe). The cooling of the gas is based on a
chemical network that follows the abundances of H+, H, H2, C+, and CO and takes
shielding into account consistently. We vary the SN feedback by comparing
different SN rates, clustering and different positioning, in particular SNe in
density peaks and at random positions, which has a major impact on the
dynamics. Only for random SN positions the energy is injected in sufficiently
low-density environments to reduce energy losses and enhance the effective
kinetic coupling of the SNe with the gas. This leads to more realistic velocity
dispersions (\sigma_HI ~ 0.8\sigma_(300-8000K) ~ 10-20km/s, \sigma_H\alpha ~
0.6\sigma_(8000-3e5K) ~ 20-30km/s), and strong outflows with mass loading
factors of up to 10 even for solar neighbourhood conditions. Clustered SNe abet
the onset of outflows compared to individual SNe but do not influence the net
outflow rate. The outflows do not contain any molecular gas and are mainly
composed of atomic hydrogen. The bulk of the outflowing mass is dense (\rho ~
1e-25-1e-24g/cc) and slow (v ~ 20-40km/s) but there is a high-velocity tail of
up to v ~ 500km/s with \rho ~ 1e-28-1e-27g/cc.
