The busiest unit in our national park system isn't a park at all. In fact, plenty of visitors never notice that they're in it — not when they're half-lost in the redwoods of Marin County's Muir Woods, not when they're deep into a conversation about robots at San Francisco's Ft. Mason, not when they're roaming the vast beach flats of Ft. Funston, near the San Mateo County line.