Motorists speeding eastward on Gratigny Parkway might not know that behind the wall of pine trees to their right, mountain bikers are hard at work. If you bring your stump-jumper there, you'll find a frondescent labyrinth winding through the woods on the park's northern edge (or "undeveloped" side, as county employees call it). The course isn't hilly, much less mountainous, but it does have a sufficient number of drops and curves over coral rock and through dense flora to provide a distinctly South Floridian challenge. Maybe we should call it jungle biking. The foliage is tunnel-like in places, so don't forget to duck! The course is home turf of the twenty-member South Florida Dirt Dobbers racing team, which hosted the second round of the recent Sandblaster Mountain Bike series.