When I was living in Washington State, I was told that there were entire towns of Spanish speakers which were not listed on maps. Social Workers and local/county government people knew where they were, but a mainstream citizen could spend their entire life unaware of their existence. (I heard about them because I did some volunteer work with the county.)

Not surprising. I guess it happens everywhere. Out here, you visit some obscure town, and as you get off the train, you realize everyone is speaking Korean or Mandarin. I guess it's a combination of family moving near family, and friends moving near friends (sometimes coupled with political ties -- e.g. the Hmong population in Minnesota). All of a sudden, you have an ethnically homogenous city in a foreign (to them, historically) country.

I suspect the Hispanic towns in Washington are also driven by cost of living. As I understand it, they are located out in the middle of nowhere. But my knowledge dates from 1992, so this may no longer be true.