  Multiple SN explosions in disk galaxies efficiently evacuate gas and form
cavities with the sizes and shapes of the surrounding envelopes determined by
the total amount of injected energy and by the initial gas distribution. Such
cavities are seen as HI holes when observed in face-on galaxies. Gas
hydrodynamics simulations are performed to obtain the quantitative
characteristics of HI holes that could serve for the determination of the gas
vertical scale height and the corresponding dark matter content and its
distribution. Among these characteristics is the ratio of the maximum column
density in the HI ring surrounding the hole to the background HI column density
and the vertical expansion velocity of gas in the HI ring. We show that in some
cases the extragalactic background ionizing radiation may produce HI holes in
the outer regions of galaxies, and can account for the existence of HI holes in
nearby face-on galaxies with the apparent lack of an underlying stellar
population.
