Devil's Den hike in Big Bend National Park begins the same way as the relatively casual hike to Dog Canyon, but it is a more difficult and exciting alternative. The Devil's Den itself is a narrow limestone canyon that appears like a knife cut in the mountainside. It seems entirely enigmatic from afar, and only by hiking to the top of it can you see the source, a perched valley along the spine of the Santiago Mountain Range. This is Dagger Flat, named for its abundance of healthy yucca plants and their blade-like leaves. Devil's Den is one of the few outlets for this large basin, which in wetter times could have been a mountaintop lake.
You don't have to hike all the way to the top to enjoy Devil's Den, but you do have to make it about 3 miles across open desert terrain to gaze into the narrows up close. It is an approximately 7-mile round-trip hike if you go to the top. Start out on the trail to Dog Canyon, which intersects a dry wash after about 1.5 miles. This is the trail junction, where a sign points left to Dog Canyon and right to Devil's Den. Follow the wash upstream for a while before taking a tributary wash to the left toward the mountain. There is no sign, but a well worn path and probably some rock cairns show the way. This wash leads eventually up into the mouth of the canyon, but it gets steep, rugged, and brushy. If you keep an eye out to your right, you may notice a path leading up out of the wash. Follow this to reach the canyon's south rim.
These are the two ways to experience Devil's Den, either from the bottom or the top. The wash takes you right into it, and the higher alternative takes you above it. The high road is easier, but the low road is more adventurous. A full-value circuit would be to take the high road to the top of the canyon, get a view over Dagger Flat, then descend through the canyon. Any hiking in the bottom is tough because of boulders, dryfalls, and lots of poky plants to scramble through.
The Chihuahuan Desert is one of the most biodiverse arid ecosystems on the continent, and this trail is a nice tour of plant varieties if you know what to look for. Even a casual observer will notice many types of cactus and yucca, and the composition changes with elevation and geology as you move up the mountainside.