A HI supershell is a superbubble (or supershell) of HI gas (HI region), presumably from supernovae and stellar winds, hundreds to thousands of light-years across. Superbubbles are cavities of hot gas (on the order of 1,000,000K) less dense than their surroundings.
The solar system is in an old superbubble known as the Local Bubble, about 300 light-years across.
The first sign of HI supershells were 21cm lines. X-ray emission from the heat is also observed.