Although a decade has passed since the discovery, the nature of water
masers from the nuclei of galaxies is not well known. The energy
emitted in the 22 GHz water rotational line for such
sources is about 100-1000 solar luminosity if isotropic
emission is assumed. A collision-collision pumping model has been
proposed to account the high luminosity of the extragalactic water
masers but it is not necessarily successful. The water maser cloud
must lie near the nucleus of galaxies. Such molecular clouds near the
nucleus must be related with nuclear activity of these galaxies as a
source of blackhole. One of intriguing phenomenon discovered
recently is extreme high-velocity water maser emission (about 1000 km/s)
in NGC 4258 (Nakai et al. 1993; Nature 361, 45). The extreme high-
velocity components can be seen at both low- and high-frequency sides
and the emission spikes spread in the width of about 200 km/s.
No emission between high-velocity and central components are observed
in the width of about 600 km/s. One of the possible models to account
this phenomena is the rotating disk model in which a gas ring rotates
with a velocity of 1000 km/s around the blackhole of about 10^8 solar
mass. The extreme velocity components come from the parts of the
ring moving toward and away from us, so that the Dopper velocity
observed is the speed of the rotation of the disk. The observations
of thermal emission of water molecules at the far-infrared wavelengths,
for example, J(K,K)=2(1,2)-1(0,1) transition at 179.52 micron,
if detecetd, restrict the size of the molecular cloud with water maser
emission and discriminate one from the other possible mechanisms for the
extreme high-velocity phenomena. For comparison, the observation of
normal water maser flare sources as IC 10 is also included.