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Based on HI maps covering the 3 600 square degrees area delimited by , we investigated the neutral hydrogen distribution in the Gum nebula and its environments. The main result is the appearence of a huge disk of neutral gas at a kinematical distance compatible with that of the nebula, and whose angular dimensions coincide with the optical image of the Gum nebula. The disk, roughly centered at , is located at and has a radius of . We propose that this HI disk is the neutral gas component of the optical nebula, hence becoming the first time in which a structure with the dimensions of the optical nebula is detected in other wavelengths. We analyze the fitting of observations with different theoretical models and conclude that the most plausible interpretation is that the Gum nebula is a supershell produced by the action of repeated SN explosions, where the explosions may have originated in the aged association Vela OB2. In this context, the thick HI shell shown by Dubner et al.(1992) near and the Vela SNR are the debris of two of the several outbursts which started in the region about years ago. The southern boundary is distorted due to the presence of a radius shell, which appears to be an interstellar bubble driven by the stellar winds of the O–type star HD 49798.

We present a high resolution spectroscopic study of the ionised gas in Puppis-Vela, which includes the Gum Nebula and the IRAS Vela Shell, in the emission lines of Halpha lambda6563 angstrom, [NII] lambdalambda6548, 6584 angstrom and the [OIII] lambda5007 angstrom. Line profiles were obtained at 18 positions well distributed over the entire region. Only the [NII] line profiles in the region of the IRAS Vela Shell show double peaked profiles, consistent with an expansion velocity of 10 +/- 2km s-1. The ionised gas associated with the 36-degrees-diameter Gum Nebula does not show any such systematic motions. This is consistent with, and provides further confirmation of, the results obtained from the IRAS Super Sky Flux (ISSA) maps (Srinivasan Sahu & Blaauw 1993) that the IRAS Vela Shell and the Gum Nebula are two separate structures but seen overlapping in projection.We detected no [OIII] lambda5007angstrom emission in any of the positions observed. From the [NII]/Halpha ratio, the temperatures derived at the 18 positions vary between 5,000 to 12,0000 K.