  Taking a clue from the pair of HI supershells found in the Scd galaxy NGC
3556 (M 108), we propose a new mechanism for the origin of HI supershells in
gas-rich massive galaxies. In this scenario, the two supershells were inflated
out of the neutral hydrogen disk due to the localised flaring of a pair of
radio lobes formed by the jets ejected from the nucleus during an active phase
about 10^7 years ago, but have faded away by now. It is shown that the salient
features of this supershell pair, such as their symmetrical locations about the
galactic centre, the anomalously large energy requirements, the large
galactocentric distances, as well as the Z-symmetric hemispherical shapes, find
a more natural explanation in terms of this scenario, as compared to the
standard models which postulate either a massive starburst, or the infall of
external gas clouds. Other possible implications of this hypothesis are briefly
discussed.
