For well over a century, the dependable Old Faithful has been the most popular geyser in the park, erupting more frequently than any of its higher or larger rivals. As a result, a half-moon of concentric benches, backed by visitor facilities including the gigantic log-built Old Faithful Inn , now surround it at a respectful distance on the side away from the Firehole River. On average, it "performs" for the expectant crowds every 78 minutes, with a minimum gap of half an hour and a maximum of two hours; approximate schedules are displayed in the nearby visitor center and in the lobby of the inn. The first sign of activity is a soft hissing as water splashes repeatedly over the rim. After several minutes, a column of water shoots to a height of 100 to 180ft, the geyser spurting out a total of 11,000 gallons.

Two miles of boardwalks lead from Old Faithful to dozens of other geysers in the Upper Basin. If possible, try to arrive when Grand Geyser is due to explode. This colossus blows its top on average just twice a day, for twelve to twenty minutes, in a series of four powerful bursts that climb to 200ft. Other highlights along the banks of the Firehole River, usually lined with browsing buffalo, include the fluorescent intensity of the Grand Prismatic Spring at Midway Geyser basin , especially breathtaking in the early evening when human figures and bison herds are silhouetted against plumes of mineral spray.

Thirty miles north of Old Faithful, in the less crowded Norris Geyser Basin , two separate trails explore a pallid landscape of whistling vents and fumaroles. Steamboat is the world's tallest geyser, capable of forcing near-boiling water over 300ft into the air; full eruptions are entirely unpredictable (the two most recent were on May 2, 2000, and October 2, 1991), but it usually delivers lesser bursts of ten to forty feet a couple of times a day. The Echinus Geyser is the largest acid-water geyser known; every 35 to 75 minutes it spews crowd-pleasing, vinegary eruptions of forty to sixty feet. The Emerald Spring is a 27ft-deep pool where the vivid blue water combines with the yellow of the crater to create a stunning, jewel-like color.

At Mammoth Hot Springs , at the northern tip of the Loop Road, terraces of barnacle-like deposits cascade down a vapor-shrouded mountainside. Tinted a marvelous array of grays, greens, yellows, browns and oranges by algae, they are composed of travertine, a form of limestone which, having been dissolved and carried to the surface by boiling water, is deposited as tier upon tier of steaming stone.

Geysers

• Geysers

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