  Several examples of thin, Keplerian, sub-parsec megamaser disks have been
discovered in the nuclei of active galaxies and used to precisely determine the
mass of their host black holes. We show that there is an empirical linear
correlation between the disk radius and black hole mass and that such disks are
naturally formed as molecular clouds pass through the galactic nucleus and
temporarily engulf the central supermassive black hole. For initial cloud
column densities below about 10^{23.5} cm^{-2} the disk is non-self
gravitating, but for higher cloud columns the disk would fragment and produce a
compact stellar disk similar to that observed around Sgr A* at the galactic
centre.
