Kilauea Caldera, from many of its overlooks, seems to be a vast place. But the entire crater is only about 11 to 12 miles in circumference. Every July, more than a thousand runners from all over the glob converge on the caldera for the annual Kilauea Volcano Wilderness Run, where hardy athletes circumnavigate the caldera in less than two hours, via a 10-mile course that includes sections of the Crater Rim Devastation and Byron’s Ledge Trails and a quarter-mile 35-degree set of switchbacks that some runners affectionately call “The Ladder” or the “Stairway to Heaven.” In addition to the 10-mile lap, there is also a marathon and a 5-mile run-walk.

But reasonably fit visitors who don’t have asthma or heart problems can circle the crater at a walk at any time, via the Crater Rim Trail, and still be back in a five to eight hours.

For runners, the caldera offers unique thrills: the unforgiving terrain, with it sulfur fumes, loose cinders, steep switchbacks, and broken-glass-sharp rocks, makes running an extreme sport (some runners lessen the risk by wearing skateboarder-style elbow and kneepads).

But walking around the caldera also has its advantages. For one thing, you can dress more sensibly, in sturdy jeans and more protective shoes – which in this case, probably means a good pair of old-fashioned leather hiking boots. You can wear a broad-brimmed hat against the tropical sun – though you still may have to grab onto it on the windy lava flows – and can carry a backpack with rain gear, instead of just getting soaked. And while the Crater Rim Trail is perhaps a mile and a half longer than the race course, there’s no ladder to climb.

But the main advantage of walking around the caldera is that you have more time to look at some of the island’s most spectacular scenery, and to notice the area’s unique wildlife, from tiny native forest plants to endangered birds.The Crater Rim Trail is like the National Park a microcosm: over the course of its 12-mile length, it passes from the cool of the summit to the windy heat of the Ka’u Desert to the tropical lushness of the rainforest, with volcanic fire pits and steam clouds thrown in for good measure.Steam vents, Halema’uma’u, the Ka’u Desert, the rainforest, Thurston Lava Tube, ash falls, Kilauea Iki, the caldera ledges: each of these features has at least one whole trail devoted to it. But only one trail, the Crater Rim, can encompass them all in one day.

The Crater Rim Trail can be accessed from several different points, including the parking lots of Hawaiian Volcano Observatory, Thruston Lava Tube and the Kilauea Iki overlook. But probably the simplest starting point is the Visitor Center. It’s wise to pick up a copy of the Halem’uma’u Trail Guide from the rangers there before you set out.The pamphlet gives a pretty good map of all the trails in the summit area, as well as descriptions of some of the plants and geological features along the trail. You should also fill you water bottles thee, if you haven’t already (three liters per person are recommended for a full day’s hiking).

It’s probably best to start early in the morning and take the trail counter-clockwise, to avoid the worst of the sun on the ash flats of the Ka’u Desert. From the Visitor’s Center, you can either cross the road and take up the trail behind Volcano House, or else strike out on the Sulphur Bank Trail at the makai end of the parking lot. The yellow-and-white Sulphur Banks are more colorful, but the trail from Volcano House offers spectacular cliff-top views of the caldera.

Either way, you’re going to encounter the first of the day’s hazards: sulfurous fumes from the many steam vents in this fissure-wracked reagion, known as the Steaming Cliffs.

This is not a great section for those with asthma or heart conditions. But then, neither is much of what follows.

The Sulphur Banks Trail soon crosses Crater Rim Road to join up with the main trail, which skirts along the edge of the cliff overlooking the caldera. Halema’uma’u, the 300-foot-deep, 3000-foot-wide traditional home of Madoame Pele, quickly comes into view. The crater merely steams today. But from 1823 to 1924, it held a roiling lake of hot lava.

The crater’s name offers a ghostly glimpse of an even earlier vista. “Halema’uma’u” probably refers to ama’uma’u tree ferns; it literally means “house of ferns.”

The barren, sulfur-streaked crater, still reeking of poisonous fumes, must have looked far different at some point in the past. Ancient Hawaiians may have first seen it in some dormant phase, when small forest of tree ferns, may have carpeted its now-desolate floor.

If you scan along the inner walls of the Halema’uma’u or watch the air along the cliffs rimming the caldera, you may see one of Hawaii’s more remarkable residents: the white-tailed tropcbirds or koa’e kea. Graceful white seabirds with elegant black wing markings and long streaming tails, koa’e kea travel miles inland to nest on remote cliffs of “high islands” such Hawaii. Apparently impervious to sulfur fumes, they love to hang out in Halema’uma’u and the caldera, where they spend hours soaring and swooping on the updrafts rising from the still-steamy crater floor.

Perched on the high cliff overlooking Halema’uma’u is the Hawaiian Volcano Observatory and the Thomas A. Jagger Museum, which is well worth a look if you’ve never been inside. Besides a wealth of exhibits explaining the volcano, the museum offers visitors a chance to watch as the dancing needles of the observatory’s seismographs record the dozens of minor earthquakes each day as lava shifts in underground reservoirs and the island settles under its own growing weight. From the museum, the trail descends along with the dwindling cliff, until it crosse the highway and enters the hot, windy lava flats and orange-brown-pink ashlands of the volcano’s Southwest Rift Zone and the Ka’u Desert.

The Rift cuts like a miniature canyon through the patchwork of black-and-grey lava flows, revealing dozens of Layers of volcanic history. Layers of ash have hardened into the closest thing that the island possesses to sedimentary rock – sometimes only to be blown apart by later eruptions, leaving huge hunks of striped rock littering the badlands.

The Ka’u Desert isn’t a true desert in terms of rainfall, through it gets nowhere near the precipitation of Kilauea’s windward rainforest – now only a scant two or three miles away. But the rain quickly seeps into porous ash and fissured lava – and fresh coats of lava periodically wipe out the rain-starved survivors. Still, small ferns and miniature ohia trees cling to the fissures.

Scrubby little ‘ohelo bushes appear, often laden with tasty berries that were once sacred to Madame Pele. The park allows visitors to sample the berries but recommends leaving most of them for the native nene geese and other local wildlife.

In the middle of this desolate stretch, the Halema’uma’u Trail branches off on the left, offering a 3.2 mile “shortcut” across the steamy middle of the caldera – although a steep climb waits at the other end.

As the trail veers eastward, the plant life grows in size and numbers, until it begins to resemble a real forest again as the trail passes sough of Keanakakoji Crater. At first, the understory is choked under a layer of ash from the 1959 Kilauea Iki eruption: what seems like young trees are actually the crowns of full-grown, half-buried ohias.

But ohia trees are, of course, uniquely adapted to cope with such hazards. Their narrow, corkscrewy profiles and tiny grey-green leaves shed falling ash readily. And the red, beard-like growths that sometimes appear high on their trunks are actually aerial roots, which may serve a dual purpose: they absorb extra moisture and nutrients from the air, supplementing what the tree’s underground roots can get from t6h fast-draining soil – and may also help the tree to re-root itself if it becomes too deeply buried in ash.

By the time the trail crosses Chain of Craters Road, it’s plunging through full-fledged rain forest. Thanks in part to Park Service efforts to control feral pigs in this area, it’s one of the lushest, healthiest rain forests left on the island.

Hapu’u and ama’uma’u ferns crowd the forest floor; scarlet apapane, grayish ’oma’o, yellow-green amakihi and wrenlike elepaio flit beneath the crowns of large ohia trees. So do common invaders such as the mejiro or Japanese whiteye.

The trail strikes civilization again at Thurston Lava Tube pullout, which sports a parking lot, paved trails and restrooms. In addition to the walk-in lava cave itself, the area is an excellent spot for bird-watching: apapane and other forest birds often flit at eye-level across the gap of the small crater where stairs descend to the lava tube’s entrance.

From Thurston, the trail crosses the road again, then skirts the rim of Kilauea Iki Crater before following Waldron’s Ledge back to Volcano House.

The last segment of the trail is the easiest, traveling along the other-wise-abandoned section of earthquake-damaged highway, with apapane singing in the trees all around.

Twelve miles is a fair amount of exercise, whether on the run or at a walk. Unless hikers are in good shape, they’re liable to be a little footsore at the end. But few trails can reward hikers with as much diversity in a few hours as does the Crater Rim Trail.