Lonely Planet review
For a hardcore experience, there's the daunting 6-mile Mauna Kea Summit Trail, starting at 9200ft and climbing almost 4600ft to the summit. Thin air, steep grades and biting weather make this a strenuous hike; plan on taking eight hours for the round-trip and bring a gallon of water per person – dehydration is a real danger at these altitudes. This hike is not recommended in inclement weather. Get a near-predawn start to give yourself the maximum hours of daylight. Before hiking, register at the visitors center (if it's closed, fill out a form and place it in the drop box provided), grab a hiking map and consult with rangers about weather and other conditions.
Park at the Onizuka Visitor Information Station and walk up the road to the Kilohana picnic area, from where you'll follow signs for the Humuʻula Trail . This uphill climb is well signposted for the first mile as it continues doggedly upward, occasionally jogging around cinder cones; avoid the false spurs leading back to the access road. The already thin vegetation starts to disappear altogether as you make your way higher and the trail is marked with red poles and reflectors.
After about an hour, the summit road comes back into full view, about 100yd to your right. Remember to take your time acclimatizing, with frequent rest and water breaks if necessary. After the two-hour point, the ascent becomes more gradual, weaving among giant cinder cones. After a leveling off through basalt flows and glacial till, the trail rises through white boulders and the summit of Mauna Kea looms far ahead.
This part of the hike passes through the Mauna Kea Ice Age Natural Area Reserve . A Pleistocene glacier once moved down this slope, carrying earth and stones that carved grooves which are still visible in the rocks. After about three hours you enter a broad valley, followed by a sharp, short ascent on crumbly soil over a rise. Keanakakoʻi , an ancient adze quarry, comes into view; look for large piles of bluish-black chips. From this spot, extremely dense and hard basalt was quarried by ancient Hawaiians to make tools and weapons used throughout the islands. Do not disturb anything in this protected area.
After a steep mile-long ascent, the trail connects with a road and a four-way junction beyond. From the junction, find the trailhead for the 10-minute detour to Lake Waiʻau . Hawaii's only alpine lake sits at 13,020ft, making it the third-highest lake in the country. The lake is only 10ft deep, yet it's never dry. It's fed by permafrost (left over from the last Ice Age) and meltwater from winter snows, which quickly evaporate elsewhere on Mauna Kea.
Back at the four-way junction, make your way north (uphill yet again) for the final upward push to the Mauna Kea Summit Rd. Suddenly the observatories pop into view and straight ahead is Millimeter Valley , nicknamed for the three submillimeter observatories on the summit. The trail ends here at the road's 7-mile marker, just below the hairpin turn; turn back here and you can descend the way you came or take the road. The latter is 2 miles longer, but faster and easier.
If you really need to place a boot toe on Puʻu Wekiu , Mauna Kea's true summit, suck it up the last 1½ miles of trail. To reach it, follow the main road up to the right (not the spur road into Millimeter Valley) for over 1 mile. Past the 8-mile marker, where the road forks, veer right and look for a 'trail' opposite the University of Hawaiʻi 2.2m Telescope observatory. This 200yd trail-of-use descends steeply east, crosses a saddle and then scrambles up to the summit.
Walking back down the way you came is an option, of course, but the trail's steep grade and crumbling cinders make for a tricky, unnerving descent, especially as daylight fades and afternoon clouds roll in. It's faster to take the paved road instead and hiker aloha means someone will probably give you a ride.