While Marin has its fair share of epic rides for the area, there's the whole continent to explore when it comes to epic mountain bike rides, so here's a list to get you started:
Moab, Utah - Everyone should ride the slickrock trail once and understand what it's like to seemingly defie gravity. Porcipine Rim is the real epic trail of the area, and Amasa Back is another highly rated, more accessible ride.
Hot Springs, AR (The Womble) Ashland, OR
Other Lists of Epic Mountain Biking Trails: