Based on HI maps covering the 3600 square degrees area delimited by 230°≤l≤290°, -30°≤b≤30°, 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 l≃262°, b≃-3°, is located at ∼500pc and has a radius of ∼150pc. 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 (266°, -2.5°) and the Vela SNR are the debris of two of the several outbursts which started in the region about 6x106 years ago. The southern boundary is distorted due to the presence of a ∼28° radius shell, which appears to be an interstellar bubble driven by the stellar winds of the O-type star HD 49798.
ISM: supernova remnants - ISM: individual objects: Gum nebula - ISM: bubbles - radio lines: ISM