  Discovered in 1995 at the Caltech Submillimeter Observatory (CSO), the
vibrationally-excited water maser line at 658 GHz (455 micron) is seen in
oxygen-rich giant and supergiant stars. Because this maser can be so strong (up
to thousands of Janskys), it was very helpful during the commissioning phase of
the highest frequency band (620-700 GHz) of the Submillimeter Array (SMA)
interferometer. From late 2002 to early 2006, brief attempts were made to
search for emission from additional sources beyond the original CSO survey.
These efforts have expanded the source count from 10 to 16. The maser emission
appears to be quite compact spatially, as expected from theoretical
considerations; thus these objects can potentially be used as atmospheric phase
calibrators. Many of these objects also exhibit maser emission in the
vibrationally-excited SiO maser at 215 GHz. Because both maser lines likely
originate from a similar physical region, these objects can be used to test
techniques of phase transfer calibration between millimeter and submillimeter
bands. The 658 GHz masers will be important beacons to assess the performance
of the Atacama Large Millimeter Array (ALMA) in this challenging high-frequency
band.
