Tamalpais is Northern California's best-loved mountain. More than 1.5 million visitors a year hike, bike, ride horses, and stand awestruck in its redwood canyons. But, remarkably, the Marin County hillside still retains a good deal of mystery. There are wild canyons, hidden routes, and discoveries in its roughly 25,000 acres, from the ridgetop to the grassy slopes and beachside coves. Here are two treks to try: Dipsea Trail, the mountain's most famous route, and Cataract Trail, a lush favorite in early spring.