  The energy source of the giant HI supershells in nearby galaxies and in our
own is still an unsettled issue. Proposed scenarios for production of
near-complete ringlike supershells are multiple supernovae (SNe) and gamma-ray
bursts (GRBs). In the late phase of evolution it is difficult to tell these
models apart. We show that, if a supershell has been powered by multiple SNe,
peculiar metal abundances should be found in the medium within the bubble. We
present line diagnostics that could detect such unusual abundances, such as
unusually high ratios of [O]/[Fe] and [Ne]/[Fe]. Among ions of the same
element, a higher enhancement is expected in lines from a high-ionization state
than in lines from a low-ionization state. Identification of the energy source
of HI supershells would set strong constraints on the rates and energetics of
GRBs, as well as on their location within a galaxy.
