South of Waimea on the Big Island, there is are old 4WD roads which climb from 5,500 feet to over 10,000 feet on the slopes of Mauna Kea. The route, occassionally signed as R-1, has numerous side roads. Make sure to always follow the most heavily travelled route.
To reach the mountain biking trailhead from Waimea, head south on SR190 for about 6 miles. Veer left continuing south on SR200 (Saddle Road). The trailhead is between milepost 43 and 44 on the left (east) side of the road. Look for the old 4WD trail marked as R-1.
The route climbs steadily for the first 4 miles on a moderately graded gravel road. Keep an eye out for wild pigs, turkeys and goats. At the Pu ula au Ranger Station, the road to the left continues wrapping around the north side of Mauna Kea for 35 miles. The other road to the right heads up steeply for another 6 miles to over 10,000 feet (it is known as the Skyline Trail).
Note: Be aware that most Hawaiian topographic maps use the Old Hawaiian Datum. When trying to use the latitude/longitude from those maps with a GPS unit, you will think you are 0.4 miles south/southeast of where you really are. Kind of important in a dense jungle.