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Twenty Mile Trail

A male hiker overlooking the massive lava vent at the end of the Twenty Mile Trail. Hells Half Acre, Idaho

The Twenty Mile Trail is a 5.1 mile (one way) hike in Hell’s Half Acre beginning at the Lava Rocks Trailhead in the Snake River Plain of southeastern Idaho. While the trail itself is 4.5 miles, it will require you to use a section of the shorter loop trail which will add about 0.5 miles each way. And while the Twenty Mile Trail is actually only 4.5 miles, it likely gets its name by being 20 miles west of Idaho Falls. It’s also frequently referred to as the Lava Rocks Trail, or even the Hell’s Half Acre Trail.

Twenty Mile Trail Description

There’s no real “trail” to speak of along the Twenty Mile Trail. In essence, you’re following large poles marking the way over a rough and unforgiving lava landscape. And while the overall elevation change isn’t terribly significant, there is plenty of up and down over small rises and dips as the rock recedes and climbs. There are no unique landmarks along the way or special features to look out for; simply follow one pole and look for the next.

That being said, there is a significant amount of route finding. Many poles have been broken in half due to weathering. Some are harder to find, hanging from a branch on a juniper for example, or too distant to see clearly right away. You’ll find it extra challenging if you’re hiking this trail in late summer or in the autumn season when the sun is low in the southern sky. The glare will only increase the difficulty in spotting the next marker.

Lava Hills Breaking Apart
Hills of lava breaking apart and crumbling along the Twenty Mile Trail. Hells Half Acre, Idaho

Keep in mind there are also plenty of hazards that you won’t find on a typical trail. The most obvious as previously mentioned is that there’s really not an official trail. It’s simply finding your way over a volcanic landscape from one marker to the next. And while here and there a faint trail can be seen worn into the rock from time to time, it’s nothing you can depend on at all. Likewise, the rocks are frequently uneven or even loose. The entire hike is exposed so expect constant sun exposure while also watching for prickly pear cacti growing all over the landscape. You’ll also have to navigate around, or over large cracks in the lava.

Despite all this, the destination to the lava vent is absolutely mind-blowing. Upon first reaching the final marker and overlooking the northern part of the vent, the landscape looks interesting from a geologic perspective, but nothing too exciting. Hiking around the vent however (the mileage for which is not included in the total), yields a remarkably violent event, appearing as though it could spew lava once again at any moment. The farther south you travel along the vent’s rim, the more chaotic and destructive the landscape appears. It’s an incredible piece of geology that would fill even those with a mild interest in geologic wonders with a flood of inspiration and awe. If the aforementioned warnings sound more like adventure to you, this is a hike you’ll want to experience.

Once you’ve explored and admired the lava vent, head back out the same way you came: one marker at a time.

Getting There

From Idaho Falls, travel west on Broadway Ave., merging with Highway 20, for 20 miles. After mile marker 287, look for the Lava Rock Trail sign and turn left. The dirt road will have a couple of forks, but continue left at each and the road will dead-end into the parking area at the trailhead after less than 0.25 miles.

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