Aird et al. 2015, in press; Harrison et al. 2015, submitted). The 22 GHz maser line emitted by water vapor molecules having the 6$_{16}$ – 5$_{23}$ rotational transition can pass through thick absorbing matter and probe the innermost part of the nuclear structure, where high density and near edge-on geometry are needed to produce maser amplification. It has been a long time since the first water vapor extragalactic maser emission was discovered in M33 . Today, nearly 200 galaxies have been detected in H$_2$O maser emission, some associated with disk structures, jets or outflows (e.g., see Table 1 of ). Because of their high luminosities with respect to Galactic masers, extragalactic water masers associated to AGN are generally called megamasers, while those associated to star-forming regions are sometimes referred to as kilomasers. Very long baseline interferometry (VLBI) radio observations provide a robust tool to study subparsec structures. In particular, in disk maser systems, precise estimates of the central dynamical mass can be performed. VLBI maser mapping has been used to test the existence of supermassive black holes (SMBHs) ruling out other candidates (such as rich clusters of stars), and to measure distances independent of a cosmological model [@2009ApJ...695..287R; @2013ApJ...767..154R see 