Measured from its base below the ocean's surface, Mauna Loa is well over 40,000 feet high, making it easily the world's second tallest (Mauna Kea, not far away is 400 feet taller). But this shield volcano (made from lava roiling up from the earth's core, bursting from the ocean surface, and making a big pile) is by far the planet's most massive, with 100 times the volume of Mt. St. Helens or Mt. Rainier.
The current Kilauea Caldera eruptions are taking place well down the southeastern flank of Mauna Loa.
The drive to the Mauna Loa trailhead near the top climbs 4,500 feet over about 18 miles. When the road was paved a few years back, workers left the first quarter mile unpaved, presumably to deter curious tourists.
The road provides smooth passage through a sea of lava from several eruptions that took place the late 1800s and in 1935. The volcano is very much considered 'active.'
To avoid altitude sickness, and for kicks, you should stop for maybe an hour on the way up and take a stroll. Pick a spot with pahoehoe (pah-hoy-hoy) lava, which lays out like brownie batter, rather that the sharp slag heaps of a'a (ah-ah) lava.
Add water and sunshine and life begins anew.
Views get big as you climb: To the north are Mauna Kea, Hualalai Volcano and Maui's Haleakala.
The Mauna Loa Observatory is at road's end, where weather stations manned by scientists from around the world conduct experiments and collect data. This is where the depletion of the ozone layer and carbon dioxide buildups have been recorded for decades. The folks don't mind if you walk up there, but don't be a pest since it is not a tourist attraction. Once-weekly tours are available, however.
With boardwalks connecting metal building and trailers, and weird installations scattered about, the observatory is the 'poor cousin' to the cityscape of gleaming celestial observatories across the saddle on Mauna Kea. Far fewer (practically zilch) visitors see Mauna Loa.
A 12-mile round-trip hike with 2,000-plus feet of gain sounds more doable than it is. Lack of oxygen is pronounced, and you can freeze and fry on the same day. Since Mauna Loa is so round, you don't see the summit as you climb. You can also approach from the south side of the peak—at the Mauna Loa Lookout—but this is not advisable these days with Kilauea blowing its cork.
Hawaii the Big Island Trailblazer has more details of exploring the volcanic highlands.