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 .