Just after Marin County planners raised the roof on their new 110,000-square-foot prison in this affluent San Francisco suburban area, they hid it under a plush layer of grass. Nearby, citizens mill about the Frank Lloyd Wright-designed Civic Center. But below more than 200 prisoners carry out sentences in 6-by-12-foot cells in a jail dug 60 feet into the earth. When they need to face a judge or jury, guards walk them through an underground tunnel to the courthouse. As far as sunlight goes, they get a glimpse while eating, thanks to the cafeteria's domed skylight. And there is no unsightly prison to tarnish the Wright structure, a prospect that had concerned local preservationists. How do conditions compare with a notorious, former island prison nearby? Undersheriff Bob Doyle insists the jail is certainly more humane--"There's no comparison to Alcatraz." It's surely less conspicuous. As Doyle says: "You can drive right by it and not even know it's there."