Years ago I knew a guy who grew up in The Dalles, Oregon, and he'd never
had a cavity (he was about 34 at the time).  He had very thick enamel on
his teeth, which looked as if they had been chiseled out of white
marble.  He told me that good teeth were common there, and that the
region got its drinking water from the Columbia River, which was very
high in calcium and magnesium.

Pat