In this paper, we have presented neutral hydrogen 21 cm images of a
region covering completely the area subtended by
the optical image of the Gum nebula. The maps have been constructed
combining data extracted from the surveys performed by Weaver & Williams \
(1973), Heiles & Habing \
(1974), Colomb et al. (1977, 1980) and Strong et al. \
(1982). The main result was the discovery of a giant HI disk which
holds the optical nebula in its interior. This is the first study in which
a structure with the same dimensions of the optical image of the Gum
nebula is detected in another spectral region. Another case of a huge HI
structure detected in connection with an optical supershell in our galaxy is
the Cyg OB1/OB3 supershell (Dewdney & Lozinskaya 1994).
We cannot unequivocally conclude that the HI disk is associated with the
optical Gum nebula. However, we believe that there is a reasonable
probability that both structures arise from the same physical mechanism. We
investigated the hypothesis of the Gum nebula being a supershell driven
by multiple supernova explosions, in the context of the theory of McCray
(1988). The distance to the Gum nebula was estimated to be 
pc, thus having a radius of 
pc. Based on a supershell
model, an age of about 
yrs was derived. A possible signature of
a "blowout'' towards the northern extreme of the shell was detected
at 
. This feature, which has the appearence of a chimney, is
consistent with the locus of convergence of the two upper 
wings and agrees with the theoretical predictions of Tenorio-Tagle et al.
(1990a).
A number of pulsars enclosed by the supershell may be indicative of
the occurrence of multiple ancient explosions in the region. Additional
support for the hypothesis of repeated supernova explosions in the past is
given by the thick HI
shell reported by Dubner et al. (1992), which was proposed to
be an old SNR based on different wavelengths results. The source of the
explosions can be attributed to the Vela OB2 association. We suggest
that the 
emission arises from the ionization of the HI disk
produced mainly by the early stars 
Pup, 
Vel and
CD-474551, located inside the supershell.
The Gum nebula appears to be an old, now decelerated supershell. If its low expansion velocity arises from the SN explosions having turned off long ago, then the bubble interior should have cooled and thus the corresponding hole in the HI distribution no longer exists (Vader & Chaboyer 1995). It must be mentioned, however, that a wind driven bubble model (Weaver et al. 1977) is marginally compatible with observations, both using kinematical and energetic arguments.
At smaller scales, it was found that the 
and the HI emission are
poorly correlated. In particular, the southern appearence of the Gum nebula in
is not reproduced in the 21 cm image, where the contours appear
distorted by another shell. We have shown that this southern shell is
consistent with an HI bubble blown by the O-type star HD 49798. Applying the
model of Weaver et al. (1977), an age of 
yrs was
obtained. It follows, then, that the Gum nebula and this southern shell
were born at the same time and may have interacted with one another, as
suggested by the deformation observed in the distribution of the neutral
gas.
Acknowledgements
We would like to thank J. P. Sivan for
providing us his 
photographs of the Gum nebula.