Lonely Planet review
The Mauna Loa Trail begins at the end of Mauna Loa Rd, off Hwy 11 between mile markers 30 and 31. All in, it's an extremely challenging (though non-technical) 19.6-mile hike that ascends 7000ft and takes at least three days, although four is better for acclimatization and exploring the summit area. Rain, fog, snow and whiteouts can all make the trail's ahu (cairns) hard to follow.
Two simple bunk-bed cabins are available on a first-come, first-served basis at Puʻuʻulaʻula (Red Hill) and closer to the summit. Potable water might be available, but must be treated; niquire at the Backcountry Office when picking up your free permit (required). Be prepared for severe winter conditions year-round, as well as wildfires and volcanic eruptions (very unlikely, but possible).
The trail begins rising through an ohia forest and above the tree line. After 7.5 miles you'll reach Puʻuʻulaʻula (Red Hill) at 10,035ft, offering views of Mauna Kea to the north and Maui's Haleakalā to the northwest. The next day, the 11.6-mile hike to the summit cabin (13,250ft) crosses a stark, stirring landscape of multicolored cinder fields, spatter cones and gaping lava fissures. Two miles before arriving at the cabin, turn left at the Mokuʻaweoweo Caldera trail junction for the summit cabin. The other fork leads onto the 2.6-mile Summit Trail , to tackle on day three.