You have to hunt down this stack of columns by descending a remote narrow road deep into Red's Meadow of the Inyo National Forest. The glacial wonder is just outside the eastern boundary of Yosemite near Mammoth Lakes. From Yosemite's Tioga Pass East Entrance, head south on Highway 395 and then make a right (west) onto Mammoth Scenic Loop. The trip takes about an hour to get to the entrance to the park.
The access road to Devils Postpile is so narrow that the park won't let you drive down it during daytime hours. You have to take a mandatory shuttle unless you meet their requirements for road privileges. Wait until after 7:00 pm and the park road is open to anyone who wants to venture out on it. Just know that you may need to back up to a wider section of road if you meet another vehicle. We were lucky... the only thing that approached was a black bear dashing across into the woods.
At the end of the road, you'll find a small parking lot outside the trailhead. The hike is short - less than a mile and fairly level so the walking was easy. The postpile comes into view suddenly, a very strange sight indeed. There are tall stone pillars ascending out of a pile of broken rock. Look closely and you'll notice the oddly hexagonal shape of the columns. A nearby interpretive sign explains:
"Molten lava and glacial ice shaped these unusual rock columns. Basaltic lava more than 400 feet deep filled this narrow valley nearly 100,000 years ago. As the lava cooled, cracks formed on the surface to release built-up tension. These cracks formed into hexagons, one of nature's most efficient and stable shapes. The cracks deepened as the interior cooled to form basaltic columns. During the last ice age, about 20,000 years ago, a glacier exposed this cliff of columns and polished the top surface. The postpile continues to be sculpted by weathering and earthquakes that break and change the formation."
Continue on the trail to Rainbow Falls, 1.25 miles one-way.