South of Hilo on the Big Island, the Kulani Forest has a network of informal mountain bike trails and old roads. The routes pass through lush, dense forest with giant ferns and huge stands of 60 foot tall eucalyptus trees. There is a 6 mile loop, though it can be difficult to follow with several new side routes that have formed.
To reach the mountain biking trailhead from Hilo, follow SR11 for 4.5 miles toward Hawaii Volcanoes National Park. Turn right on Stainback Highway for 2.5 miles. Turn left on Quarry Road for 0.3 miles. Look for a side road on the left leading into an abandoned rock quarry which is used as a parking area. It looks very overgrown. There is no formal trailhead or any services.
Overall, Kulani Forest Trail System is the 11th most popular mountain bike trail of all 22 mountain biking rides in Hawaii.