Neutral atomic hydrogen (H) traces the interstellar medium (ISM) over a broad range of physical conditions. Its 21-cm emission line is a key probe of the structure and dynamics of the Milky Way Galaxy. About 50 years after the first detection of the 21-cm line the exploration of the H distribution of the Milky Way has undergone a true renaissance. This was triggered by several large-scale 21-cm surveys that became available within the past decade. New all-sky surveys unravel the shape and volume density distribution of the gaseous disk up to its borders. High-resolution Galactic plane surveys disclose a wealth of shells, filaments, and spurs that bear witness to the recycling of matter between stars and the ISM. All these observational results indicate that the H gas traces a dynamical Galactic ISM with structures on all scales, from tens of astronomical units to kiloparsecs. The Galaxy can be considered to be a violent, breathing disk surrounded by highly turbulent extra-planar gas.


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  • Article Type: Review Article
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