Extremely high velocity H2O maser emission offset by 1000 km s-1 from the systemic velocity was detected in the active nucleus of the LINER NGC 4258 using the 16,000-channel AOS backend of the 45 m telescope. A detailed mapping with VLBA (NRAO) revealed a massive black hole of 3.6e7 Mo and a thin Keplerian disk of inner radius 0.13 pc around it (in collaboration with Smithsonian and NRAO groups).