For years it was believed to be a collapsed salt dome (hence the name), but later research, starting with Eugene Shoemaker, shows that nearly all the evidence points to an impact origin. Recent (2008) research by Thomas Kenkmann and Elmar Buchner have finally put the matter to rest, finding planar deformation features in quartz crystals embedded in the Kayenta sandstone of Upheaval Dome's outer ring. Here's their paper in GeoScience World: Upheaval Dome, Utah, USA: Impact origin confirmed.
(I hope someone tells the national park service; their sign at the overlook still emphasizes the salt-dome theory.)

That it is an impact crater should come as no surprise to lunar observers, since Upheaval looks remarkably like a terrestrial version of the Orientale Basin, which we see in profile on the southwestern limb of the moon, or like the Nectaris basin which is more subtle than Orientale, but has the advantage of being visible face-on from earth.

Compare this aerial photo of Upheaval Dome:	with this face-on satellite shot of Orientale:

See the similarity?

For further reading, Kurt Fisher has an excellent Bibliography of Upheaval Dome references.

Hiking Upheaval Dome

Two later trips

	Around the South Rim of Upheaval Dome
	Upheaval Canyon and the center of the dome: a hike up into the center of the crater

Our original trip

The first time we encountered Upheaval Dome, after spending a while at the overlook staring amazed at the spectacle, we attempted to hike the trail that winds downhill just inside the first shock ring to the bottom, where a spur trail heads inside the crater. Unfortunately, we ran out of water before we got much more than halfway to the bottom. The trail continues down toward this canyon (carved by a tributary of the Green River) then bends around the bottom part of the outer shock ring -- the big red rocks looming just right of center -- and the trail to the center would be just on the other side of those rocks.

Looking up from different vantage points at various parts of the outside of part of Upheaval Dome's first shock ring.

Left, another view of the outer shock ring, and the way the terrain beyond it drops off into the canyon; right, concentric shock rings.

Other links on Upheaval Dome, including some great aerial shots showing the double-ringed basin structure:

• The Controversy Over Utah's Upheaval Dome by Matt Morgan An excellent overview of the debate on the structure's origin, with an aeriel photo which shows the structure in great detail
• Impact Deformation at Upheaval Dome, Revealed by Seismic Profiles A detailed seismic study to determine whether salt exists underneath (answer: apparently not)
• A color-enhanced photoshowing topology of the area, with a link back to a page cataloguing other terrestrial impact structures