  We show that collisions between the outer Galactic HI disk and the leading
arms (LAs) of the Magellanic stream (MS) can create giant HI holes and
chimney-like structures in the disk. Based on the results of our N-body
simulations on the last 2.5 Gyr evolution of the Large and Small Magellanic
Clouds (LMC and SMC, respectively) interacting with the Galaxy, we investigate
when and where the LAs can pass through the Galactic plane after the MS
formation. We then investigate hydrodynamical interaction between LAs and the
Galactic HI disk (``the Magellanic impact'') by using our new hydrodynamical
simulations with somewhat idealized models of the LAs. We find that about 1-3%
of the initial gas mass of the SMC, which consists of the LAs, can pass through
the outer part (R=20-35 kpc) of the Galactic HI disk about 0.2 Gyr ago. We also
find that the Magellanic impact can push out some fraction (~1%) of the outer
Galactic HI disk to form 1-10 kpc-scale HI holes and chimney-like bridges
between the LAs and the disk.
