Megamasers provide a powerful tool to probe the innermost few parsecs of active galactic nuclei (AGNs). In particular, for AGNs with large inclination angles, only maser sources enable to directly study the structure and dynamics of the region inside the supposed obscuring tori, which are considered to be optically thick from the infrared to the X-ray bands. For example, the VLBI observations of the water megamaser sources at the center of NGC 4258 have provided not only the compelling evidence

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... for the presence of a central supermassive black hole but also the detailed information on the spatial and velocity distribution of the maser sources (Miyoshi et al. 1995; Nakai et al. 1995).