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The clumpy maser disks observed in some galactic nuclei mark the outskirts ofthe accretion disk that fuels the central black hole and provide a potentialsite of nuclear star formation. Unfortunately, most of the gas in maser disksis currently not being probed; large maser gains favor paths that arecharacterized by a small velocity gradient and require rare edge-onorientations of the disk. Here we propose a method for mapping the atomichydrogen distribution in nuclear disks through its 21cm absorption against theradio continuum glow around the central black hole. In NGC 4258, the 21cmoptical depth may approach unity for high angular-resolution (VLBI) imaging ofcoherent clumps which are dominated by thermal broadening and have the columndensity inferred from X-ray absorption data, ~10^{23}/cm^2. Spreading the 21cmabsorption over the full rotation velocity width of the material in front ofthe narrow radio jets gives a mean optical depth of ~0.1. Spectroscopicsearches for the 21cm absorption feature in other galaxies can be used toidentify the large population of inclined gaseous disks which are not masing inour direction. Follow-up imaging of 21cm silhouettes of accelerating clumpswithin these disks can in turn be used to measure cosmological distances.

21cm Absorption by Compact Hydrogen Disks Around Black Holes in Radio-Loud Nuclei of Galaxies