Several neutral hydrogen ( HI ) cavities have been detected in the Milky Way and other nearby star forming galaxies .
It has been suggested that at least a fraction of them may be expanding supershells driven by the combined mechanical feedback from multiple supernovae occurring in an OB association .
Yet most extragalactic HI holes have neither a demonstrated expansion velocity , nor an identified OB association inside them .
In this work , we report on the discovery of an unbroken expanding HI supershell in the nearby spiral galaxy M101 , with an UV emitting OB association inside it .
We measure its size ( 500 pc ) and expansion velocity ( 20 km/s ) by identifying both its approaching and receding components in the position-velocity space , using 21 cm emission spectroscopy .
This provides us with an ideal system to test the theory of supershells driven by the mechanical feedback from multiple supernovae .
The UV emission of the cluster inside the supershell is compared with simulated spectral energy distribution of synthetic clusters of the appropriate age ( \sim 15 Myr ) .
The observed UV flux is found to be consistent with an association of the appropriate mass ( \sim 10 ^ { 5 } M _ { \odot } ) and age required by the energy budget of the supershell .
Properties of this supershell and another previously reported in the same galaxy are used to infer its neutral hydrogen scale height and mean neutral hydrogen density in the disk .
The presence of another UV emitting stellar association in over-dense swept up gas is discussed in the context of propagating star formation .