Northwest of Hilo on the Big Island, the Mana Road is a rough jeep trail that starts in the north near Waimea and wraps around the eastern flank of Mauna Kea. This 43 mile route climbs from around 3,500 feet to over 8,000 feet from the start in Waimea to the end near the Mauna Kea Visitor Center. The routes starts on a well graded dirt/gravel road, but by the middle of the route up on Mauna Kea there are several deeply rutted areas and a few bypass roads to get around washouts. Along the way, you will be treated to awesome vistas of green pastures, dense jungle, and idyllic landscapes.
To reach the dirt biking at the northern trailhead from Waimea, head east on Mamalahoa Highway (SR19) toward Hanokaa. Near milepost 55, Mana Road will start on the right or southeast side of the road. Turn right on Mana Road and continue to first intersection. Turn left on Mana Road and you will see a turn out on the righthand side that many use as a temporary parking area.
To reach the southern trailhead from Saddle Road, go to near milepost 28. Turn north on John Burns Way (may be unsigned) heading up toward the Mauna Kea Visitor Center for 2 miles. Turn right heading east on a dirt road (may be signed with a different road name, but turns into Mana Road).
The route starts with 18 miles of smooth dirt road. Then 5 miles of heavily rutted doubletrack through tall grasslands and rolling hills. There are several side-routes to bypass the really nasty sections. Most return to Mana Road. Typically takes 5 hours.
Note: Avoid if very rainy due to mud. Also, the County states this is public right-of-way, even if signed contrary.
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.
Note: according to the county, Mana Road is a county road open to public access. However, many local landowners along the road may feel differently, including the state! You may find No Trespassing sign put up by landowners and by the state. It is up to you who you believe and trust.