Cosmology With High-Redshift Water Masers
  Multi-epoch VLBA observations of the maser in NGC 4258 have yielded a 4% geometrical distance to the galaxy. The potential scientific payoffs of finding similar objects at large distances, in the Hubble flow, are considerable. In this contribution, I discuss the plausibility of detecting high-redshift water masers, and describe a search strategy that we have implemented to realize this objective. 
J. R. Herrnstein