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Published online by Cambridge University Press: 03 August 2017
The instrumental requirements and current observational results are outlined for the study of resolved neutral hydrogen structures in both emission and absorption in external galaxies. Neutral super-shell structures exterior to ionized shells around OB associations seem to be a common phenomenon. Further analysis of existing data should allow quantification of the degree and rate of energy deposition in the ISM by massive stars. HI absorption studies offer great promise in constraining the physical properties of the neutral ISM, including an estimate of the gas pressure. Together these data will provide hard constraints on the existence and prevalence of chimney formation, and hence disk – halo interactions in a variety of galactic environments.