The Gum nebula is the largest 
emission object in angular
dimensions detected in our Galaxy, and extends over most of the Vela-Puppis
region in the southern sky. From 
images, it is described as a
diameter sphere centered at 
(Chanot & Sivan 1983). Distance estimates range from 300 pc
(Franco 1990) to 800 pc (Sahu 1992).
The nature of the Gum nebula is still a controversial matter. The models proposed to explain its origin include a fossil Strömgren sphere (Brandt et al. 1971), a classical H II region (Gum 1956; Beuerman 1973), a supernova remnant (Reynolds 1976a,b), a wind driven bubble (Weaver et al. \ 1977), a supershell produced by a combination of stellar winds and supernovæ explosions (McCray 1988) and a "blister'' in the near edge of a giant molecular cloud caused by the Vela R2 association (Sahu 1992).
There have been some attempts to determine whether or not the gas
associated to the Gum nebula is in expansion. Based on 
,
[N II], [O IV] and He I emission lines, Reynolds (1976a)
concludes that the Gum nebula is expanding with a velocity of 20 .
However, Wallerstein et al. \
(1980) and Hippelein & Weinberg (1975) could
not arrive to the same conclusion based on Ca II and Na I optical
data. Srinivasan et al. (1987) found an expansion velocity of
\
based on [N II] and 
data.
The first insight into the distribution of neutral gas in the Gum nebula has
been performed by McGee et al. (1963, 1966). They found a
ridge of enhanced emission at zero velocity following closely the 
circular envelope. Dubner et al. (1992) mapped the area
delimited by 
combining observations performed with the 30 m IAR
radiotelescope, and data from the HI 21 cm line survey of Strong et al. \
(1982), taken with the 64 m Parkes radiotelescope. Based on
these images, they show the existence of a 
radius thick
shell at a distance compatible with that of the Gum nebula. The shell,
centered at (
) and expanding at 
,
appears poorly correlated with the optical emission.
In this paper, the study is extended to the area delimited by 
so as to investigate the
characteristics of the neutral gas in the whole nebula and its surroundings in
search of a possible neutral gas counterpart for the optical nebula.
For this purpose, we combined the surveys performed by
Weaver & Williams
(1973), Heiles & Habing (1974), Colomb et al. (1977,
1980) and Strong et al. (1982) in the HI 21 cm line. The
procedure to obtain the maps was not straightforward because of the
irregular sampling of the sky for latitudes 
. In Sect.
2, the observational data bases and the procedure followed for the
construction of the maps are described. Based on the present data, possible
explanations for the origin of the Gum nebula are investigated.