  By comparing radial velocities of radio bright compact HII regions with their
HI absorption profiles, we discovered expanding shells of neutral hydrogen
around them. These shells are revealed by absorption of the radio continuum
emission from the HII regions at velocities indicating greater distances than
the observed radial velocity. We believe that these shells are shock zones at
the outer edge of the expanding ionized region. Additionally we found evidence
for a velocity inversion inside the Perseus arm caused by a spiral shock, which
results in a deep absorption line in the spectra of compact HII regions behind
it.
