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Yellowstone National Park attracts more than 4 million visitors per year as the fifth-most visited national park in the U.S. People come to this 3472-square-mile tract of Western wilderness for its towering mountains, charismatic megafauna, and, of course, its spectacular geysers. While there are loads of hydrothermal features to see, the most iconic geyser by far is Old Faithful, named by explorers in 1870. Here are a few facts about this Wyoming wonder.

Theres a longstanding myth that Old Faithful erupts every hour. According to the National Park Service (NPS), this has never been true. On a typical day, Old Faithful shoots out a plume of superheated steam around 17 times, anywhere from 60 to 110 minutes apart. Geologists can use the duration of the latest outburst and other factors to determine how long it may take for the next one to arrive. The formulas success rate speaks for itself: 90 percent of Old Faithfuls eruptions take place within 10 minutes of these calculated predictions.

Geysers occur when a subterranean reservoir of water is warmed enough to burst through Earths crust. Below Yellowstone National Park lie two volcanic chambers loaded with molten rock called magma. On top of the chambers are cavities of groundwater. The magma heats the underground water and pressure builds until water bubbles out of the ground, releasing some of the pressurewhich causes vigorous boiling and a subsequent explosion of water. Thats when the fun starts.

A geysers underground dimensions play an important part in its eruptions. The narrow reservoirs constrict the water in a tight space, while the overhanging rock creates pressure. The water at the bottom of the geyser experiences additional pressure because the cooler water nearer the top of the reservoir pushes down on it. These forces keep the low-lying water liquid, even as it grows hotter and hotter. Within Old Faithful, groundwater can hit temperatures as high as 244F. Despite exceeding waters normal boiling point at sea level of 212F, this water doesnt transform into gasinstead, it becomes superheated.

The built-up steam and boiling water reacts to the geysers internal pressures by shooting dramatically out of the Earth. Each eruption can emit 3700 to 8400 gallons of water skywards.

There are more than 500 geysers among over 10,000 hydrothermal structures in Yellowstone National Park. Geologists divide them into nine clusters called geyser basins. Old Faithful is the centerpiece of the Upper Basin in the parks western half, which contains at least 150 individual geysers. That makes it the biggest concentration of geysers on planet Earth.

Native people have inhabited what is now Yellowstone National Park since time immemorial. The Crow named the hydrothermal areas of what is now Yellowstone National Park the land of the burning ground or land of vapors, according to historian Lee H. Whittlesey [PDF]. [The] Crows specifically called the Yellowstone geysers Bide-Mahpe, meaning sacred or powerful water, Whittlesey writes. The Flatheads and Kiowas names for the parks geyser region translate to smoke from the ground and the place of hot water, respectively.

The first privately funded American expedition to the Yellowstone area took place in 1869, and the explorers published an updated map of the region. That encouraged another, larger expedition the following year, led by former Congressman Henry D. Washburn and businessman Nathaniel P. Langford with a military escort by Gustavus C. Doane. Langford described a perfect geyser It spouted at regular intervals nine times during our stay, the columns of boiling water being thrown from 90 to 125 feet at each discharge, which lasted from 15 to 20 minutes. We gave it the name of Old Faithful.

Today, feeding a bear at Yellowstone National Park can earn you a $5000 fine and a six-month prison sentence. In the past, though, the parks attitude was far more lax. Before World War II, Yellowstone managers would encourage bears to dine at purpose-built trash dumps, which attracted huge crowds of visitors. One of these heaps, located near Old Faithful, was marked with a wooden sign that said Lunch Counter for Bears Only.

That honor goes to the Steamboat geyser, a monster capable of launching 300-foot columns of water and steam. By comparison, the tallest plumes from Old Faithful are around 184 feet high. But its harder to plan a visit around Steamboats eruptions. It once went dormant for 50 years, and though its recently become more active, it still erupts far less often than Old Faithful.

In 1959, 1983, and 1998, earthquakescaused dips in the geysers daily eruption average. As the NPS explains, the average interval between eruptions has been lengthening during the last several decades. Thats correlated to the durations of the outbursts themselves. If an eruption lasts for more than 2.5 minutes, youll probably have to wait an hour and a half to watch the next one. But shorter eruptions mean shorter wait periods. And in recent years, lengthy eruptions have become more common.