Mountain biking came of age in the steep, heavily wooded areas of Marin County, but that seemed like a long time ago. There’s now a small group of uber-extreme off-road daredevils who seek out remote areas of the world where they challenge the limits of gravity and common sense. “Where the Trail Ends” is a new documentary film that was bankrolled by Red Bull and some others. Riders trek in or get helo’d into mountainous regions with their bikes. Then it’s all about biking “big mountains.” At around the 2-minute mark of this must-see film trailer is when the action really kicks into high gear. Planet Wild has never seen off-road footage like this before.