Giant HI Hole inside the 3-kpc Ring and the North Polar Spur - The
  Galactic Crater -
  Applying a newly developed tangent-circle method (TCM), we derive a volume
density map of HI gas in the inner Galaxy as a function of galacto-centric
distance $R$ and height $Z$. The HI hole around the Galactic Center (GC) is
shown to have a crater-shaped wall, which coincides with the brightest ridge of
the North Polar Spur and emanates from the 3-kpc expanding ring. The crater
structure is explained by sweeping of the halo gas by a shock-wave from the GC.
The unperturbed HI halo outside 3 kpc is shown to be in hydrostatic
equilibrium, obeying the sech$^2 Z/h$ density law with a scale height $h \sim
450$ pc.
