  The "Carina Flare" supershell, GSH 287+04-17, is a molecular supershell
originally discovered in 12CO(J=1-0) with the NANTEN 4m telescope. We present
the first study of the shell's atomic ISM, using HI 21 cm line data from the
Parkes 64m telescope Southern Galactic Plane Survey. The data reveal a gently
expanding, ~ 230 x 360 pc HI supershell that shows strong evidence of Galactic
Plane blowout, with a break in its main body at z ~ 280 pc and a capped
high-latitude extension reaching z ~ 450 pc. The molecular clouds form
co-moving parts of the atomic shell, and the morphology of the two phases
reflects the supershell's influence on the structure of the ISM. We also report
the first discovery of an ionised component of the supershell, in the form of
delicate, streamer-like filaments aligned with the proposed direction of
blowout. The distance estimate to the shell is re-examined, and we find strong
evidence to support the original suggestion that it is located in the Carina
Arm at a distance of 2.6 +- 0.4 kpc. Associated HI and H2 masses are estimated
as M(HI) ~ 7 +- 3 x 10^5 Msol and M(H2) ~ 2.0 +- 0.6 x 10^5 Msol, and the
kinetic energy of the expanding shell as E ~ 1 x 10^51 erg. We examine the
results of analytical and numerical models to estimate a required formation
energy of several 10^51 to ~ 10^52 erg, and an age of ~ 10^7 yr. This age is
compatible with molecular cloud formation time-scales, and we briefly consider
the viability of a supershell-triggered origin for the molecular component.
