This 7-mile (one-way) undulating hike over rugged terrain begins with the excellent Mauna Ulu Eruption Trail before continuing across acres of pahoehoe (smooth-flowing lava) to the edge of the mile-long Makaopuhi Crater. There it heads into the muddy fern forest past the Old Pulu Factory to the edge of Napau Crater with Puʻu ʻOʻo steaming silently on the horizon.
We have a love-hate relationship with this trail which can be grueling at times, but pays big dividends in terms of diversity and views.
Self-register at the trailhead off Crater Rim Dr for day trips beyond Puʻu Huluhulu. To overnight you'll need a permit ($10) from the Backcountry Office, where you can also ask about trail conditions and closures in this geologically unstable area. The small official camping area has no view and no water; skip it if you can manage the 14-mile round-trip in a day.
You can shave off about 4 miles if you start on the Naulu Trail from Kealakomo on Chain of Craters Rd. However, it's a rough uphill hike and you'll miss the grand views and huffing vents along Mauna Ulu's flank.