The imposing Lower and Upper Terraces of Mammoth Hot Springs are the highlight of the Mammoth region. An hour’s worth of boardwalks wind their way between ornate and graceful limestone pools, ledges and plateaus. Palette Springs (accessed from the lower parking lot) and sulfur-yellow Canary Springs (accessed from the upper loop, 1km south) are the most beautiful sites, but thermal activity is constantly in flux, so check the current state of play at the visitor center.
The famously ornate travertine formations that characterize the lower terraces of Minerva Spring have dried over the years due to earthquake activity but are still among the area's most picturesque. Nearby Mound Spring currently has the most beautiful colors and abstract patterns on the terraces. The landscape is so otherworldly that it provided the pre-CGI backdrop for the planet Vulcan during the filming of the 1979 Star Trek movie.
The terraces are the product of dissolved subterranean limestone that is continuously deposited as the spring waters cool on contact with air – over a ton of travertine comes to the surface every year. The yellow, orange and brown runoff from the naturally white terraces is due to the different bacteria and algae that flourish in the warm waters.
At the bottom of the terraces, by the parking area, is the phallic, dormant 36ft-high hot-spring cone called Liberty Cap, apparently named after the hat style worn during the French Revolution. The former hot spring must have had particularly high water pressure to create such a tall cone during its estimated 2500-year life span.
Across the road, Opal Spring is slowly converging on a century-old residence designed by Robert Reamer (the architect of Old Faithful Inn and Roosevelt Arch). Park strategists have to decide which to preserve – the architecture or the spring. The rutting Rocky Mountain elk that sometimes lounge on Opal Terrace in fall are a popular photo subject.
A 1.5-mile, paved one-way road loops counterclockwise around the Upper Terraces, 1km uphill from Mammoth; vehicles longer than 25ft will have to park on the main Grand Loop Rd. The overlook affords impressive views of the Lower Terraces and Fort Yellowstone and offers access to Canary Springs and New Blue Spring. Highlights further around the road loop include the spongelike Orange Spring Mound and the perfectly named White Elephant Back Terrace. The loop rejoins the main road near the large Angel Terrace. Rangers lead a 90-minute walk around the Upper Terraces daily at 9am.
For a private perspective on foot, walk the Howard Eaton Trail from Orange Spring Mound (unsigned) down to the lower Mammoth Terraces. Even better, start from the unsigned Snow Pass trailhead pullout two minutes' drive south from the Upper Terraces.