The Upper Terrace Drive at Mammoth Hot Springs gives access to travertine formations and hot springs southwest of the lower terraces. It is a one-way loop road open to cars, bicycles, and hikers. During the main season parking in front of the different features is often quite tricky because space is limited. Therefore, it is always a good idea to hike the 1.5 mile loop.
In July 2015 the National Park Service temporarily closed Upper Terrace Drive to vehicles due to a new hot spring that had emerged in April 2015 from a fissure next to the parking lot just across the road west of Cupid Spring. Activity of this spring continued with interruptions into early 2016. A look into the history of Mammoth Hot Springs reveals that this is a reincarnation of nearby Baby Spring, which was intermittently active during the years 1932-65. It is also the namesake of Baby Terrace, a sub-terrace of Prospect Terrace and the support structure of the current parking lot.
Investigations with the help of drillholes and thermal imaging infrared cameras showed that the feature's hot water conducting veins are continuing underneath the pavement of the parking lot. This may necessitate closures or reconstruction work in the area also in future. Interestingly, on the pictures it can be seen that the outflow channels of the spring and the drillhole are already densely populated by thermophiles, raising the question of how they can arrive so quickly. A possible answer may be the presence of a "deep, hot biosphere" in subsurface environments, a concept developed 1992 by Thomas Gold and supported by many research results since then.
West of Baby Terrace the Loop Road passes Narrow Gauge Terrace. Close to the road you find a sign with its name, but apart from this there is not much to see. Although the namegiving fissure ridge extends from here in northwestern direction, it is partly obscured by other travertine formations and so quite hard to overlook. The definitely best viewpoint can be reached on the Howard Eaton Trail. Narrow Gauge Terrace got its name because the elongated shape of the main fissure ridge resembles an old-time narrow-gauge railroad track bed. As with most fissure ridges it is characterized by a crack that extends along the top of the ridge for nearly its entire length, caused by a tension fracture. The aprons around vents of Narrow Gauge Springs are well know to microbiologists for hosting almost uniform communities of white, filamentous Sulfurihydrogenibium yellowstonense, hyperthermophilic bacteria of the Aquificales group. In the temperature range of 70 °C to 74 °C (158 - 165 °F) prevailing in the vents they are obtaining energy via metabolism of sulfur compounds. The white streamers, which are recognizable on the images of New Baby Spring and of the drillhole near New Baby Spring shown above are very likely formed of Sulfurihydrogenibium bacteria, too.
The vaguely pyramidal shaped terracette assembly of Cheops Mound shows up not far southeast of the Narrow Gauge Fissure Ridge. For the early history of Yellowstone only one recorded activity around 1904 is known from the Cheops Mound Fissure Ridge. The more surprising was a reactivation in the early 2000's, when additional springs came to life in a forested area. Since then many trees died and an impressive new mound has grown up. Because this new structure does not exactly match the position of the historical Cheops Mound terracettes some feet further east, it is more like a "Chephren Mound". In 2017 hot spring activity suffered an almost complete standstill and shifted towards the near Narrow Gauge Fissure Ridge, where many active springs appeared during that year.
At the next stop along Upper Terrace Drive a sign shows the name Prospect Terrace. This may be a little bit confusing because Prospect Terrace is an expanded area and comprises The Esplanade, Cupid Spring, Baby Terrace, and the Prospect Springs. The weathered sinter cone next to the loop road is the remains of one of the Prospect Springs. They ceased flowing at the end of the 1960s.
New Highland Terrace was formed by different springs from 1928 through to the 1980s in a forested area. Some of the embedded trees can still be seen today. New Highland Terrace is only a part of the extensive Highland Terrace, where some prominent springs were located such as Highland Spring, which was intermittently active from 1871 until 1970, and Cedar Tree Spring, which showed discharge from 1961 to the early 1970s. Probably around 2011 new and by now unnamed springs developed just in the middle of the northern edge of New Highland Terrace, whose runoffs are visible from the road. The strongest one is shown on the next picture. If you look very closely, you may spot a little gas driven water fountain on top of the spring.
West of the active springs some old, partially broken terracette deposits of former springs line the edge of the terrace.
In the south of New Highland Terrace Orange Spring Mound appears. It is an intermittently active hot spring supported by a fissure ridge. Due to the low water discharge it is growing quite slowly and has built up the characteristic cone shape over a long period of time.
Tangerine Spring is one of the cones of Orange Spring Mound, but has gotten an own name.
Across the road from Orange Spring Mound the small travertine cone of Little Burper shows up. From a vent on top often a carbon dioxide driven fountain of a few inches height could be seen before the spring ceased flowing at the end of the 1990s.
Two curves past Orange Spring Mound you can spot Aphrodite Terrace at some distance on the lefthand side (east) of the road. At this location flowing water was observed for the very first time in 1978, gradually building up terrace structures, which were named by Yellowstone's historian Lee H. Whittlesey. After some time of dormancy the Aphrodite Springs came back into action around 2013.
On a closer look the Aphrodite Springs display breathtaking terracette deposits with nice colors.
Across the road from Aphrodite Springs the dry basin of Bath Lake is still recognizable. Many times in history Bath Lake ran dry and got refilled, for the last time it contained water from 1959 to 1984. Its moderate temperatures between 38 and 64 °C (100 -147 °F) tempted many visitors to go swimming, hence the name. Even if Bath Lake was quite shallow and swimming was permitted in the old times, it did not prevent bathers from drowning. At least two men drowned in Bath Lake, one in 1889 and the other one in 1898.
Another fissure ridge farther south is poetically called White Elephant Back Terrace. Usually, the White Elephant Back Springs are far less active compared to the ones on Orange Spring Mound. So there are only minor traces of thermophilic organisms visible.
Near the end of the Upper Terrace Drive Angel Terrace comes into view. The Angel Springs are known to be intermittently active from the 1870s until 1953. Then the springs discharge declined dramatically, and the reduced activity shifted between some smaller spots.
Between the north face of Angel Terrace and the south face of New Highland Terrace the Glen Springs lie on a hidden fissure ridge. The currently most active spring in this area, which probably belongs to the Angel Terrace, can be best spotted from the parking lot at Baby Terrace (above Main Terrace).
As in the case of the Glen Springs, Upper Terrace Drive does not permit a look at all features lying to the left and the righthand side of the road. Springs such as Squirrel Springs, Poison Spring, Devil's Kitchen Springs and others remain obscured to normal visitors, and information about their current status is very hard to come by.
To the southwest Pinyon Terrace is the last hydrothermally active part of the Upper Terraces Area, which ascends farther beyond towards the summit of Terrace Mountain.