Tracing the Universe's Most Abundant Atom with the World's Largest Filled-Aperture Telescope

Among present-day observatories, the Arecibo Radio Telescope represents an extension of Galileo's vision to its logical extreme. With a diameter of 305 metres and state-of-the-art instrumentation, Arecibo continues to build on its legacy of world-class scientific achievement in radio astronomy. This paper highlights milestones in the remarkable history of this telescope, and also discusses current surveys that are imaging the hydrogen content of our Milky Way Galaxy, and far beyond, in unprecedented detail.