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 1023.5 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.