  While studying extraplanar neutral hydrogen in the disk-halo transition of
the inner Galaxy we have discovered what appears to be a huge superbubble
centered around l ~ 30 deg, whose top extends to latitudes > 25 deg at a
distance of about 7 kpc. It is detected in both HI and Halpha. Using the Green
Bank Telescope of the NRAO, we have measured more than 220,000 HI spectra at 9'
angular resolution in and around this structure. The total HI mass in the
system is ~ 10^6 Msol and it has an equal mass in H+. The Plume of HI capping
its top is 1.2 x 0.6 kpc in l and b and contains 3 x 10^4 Msol of HI. Despite
its location, (the main section is 3.4 kpc above the Galactic plane) the
kinematics of the Plume appears to be dominated by Galactic rotation, but with
a lag of 27 km/s from corotation. At the base of this structure there are
``whiskers'' of HI several hundreds of pc wide, reaching more than 1 kpc into
the halo; they have a vertical density structure suggesting that they are the
bubble walls and have been created by sideways rather than upwards motion. They
resemble the vertical dust lanes seen in NGC891. From a Kompaneets model of an
expanding bubble, we estimate that the age of this system is ~ 30 Myr and its
total energy content ~ 10^53 ergs. It may just now be at the stage where its
expansion has ceased and the shell is beginning to undergo significant
instabilities. This system offers an unprecedented opportunity to study a
number of important phenomena at close range, including superbubble evolution,
turbulence in an HI shell, and the magnitude of the ionizing flux above the
Galactic disk.
