Around here, in the strange hissing interior of the world's oldest national park, shaking, rattling, and rolling are an accepted part of the landscape.
Hundreds of small earthquakes, many undetectable by tourists prowling the surface, swarm Yellowstone every year.
But on Tuesday, a pair of backpackers, camped illegally in the Norris Geyser Basin, were treated to a jolting event truly rare and momentous: the awakening of the tallest natural geyser on the planet, which had been dormant for almost a decade.
Just as Yellowstone's famous grizzlies slumber through frigid winters until spring, many of the park's 10,000 geothermal wonders - geysers, bubbling multicolored hot springs, and steam vents called fumeroles - have been known to take extended siestas.
Steamboat Geyser, which can cast a plume of hot water higher into the sky than the length of a football field, had lain quiet for nine years, leaving geologists wondering if it would ever erupt again.
Scientists received their answer with a punctuation mark this week, in a display that served as a reminder of the mysteries surrounding earth's convoluted "natural plumbing system."
"When Steamboat erupts, there is nothing that compares to actually being there," says Yellowstone naturalist Ann Deutch, who experienced the aftermath of the two-hour water-and-steam eruption.
"There are some things in nature that are just so awesome," she enthuses. "Steamboat is one of those things that you can't describe in words. This is something that exists at the edge of the human imagination."
Initially, the eyewitness backpackers snoozing near the edge of Steamboat's mineralized crater thought the park was being beset by a major earthquake as the ground rumbled beneath them. They fled with bedrolls that had been sprawled beneath the stars.
But upon turning around, they saw a rising cloud of steam pouring 500 feet into the new dawn, a prelude to Steamboat's sudden Rip Van Winklish revival.
Prior to 1991, Steamboat had been relatively active, erupting three times in 1989 and once in 1990, but during a 50-year span between 1911 and 1961, it had been silent, according to park historian Lee Whittlesey.
Some believe that Steamboat first sprouted in 1878, six years after Yellowstone was set aside by act of Congress. Ken Pierce, a federal geologist with the US Geological Survey, notes that Steamboat's awakening in 1961 may have been linked to the 7.5 magnitude earthquake at nearby Hebgen Lake in 1959.
"If you're ... there for Steamboat's eruption, what most people remember is the roar," Ms. Deutch says. "But it's a sound that happens at a low pitch; it vibrates the eardrums. You experience it with a sense of feeling rather than hearing. The water phase of the eruption comes out so fast your eyes can't follow it."
Untold thousands of gallons of water along with high-pressure steam, mud, and pebbles were discharged in a matter of seconds, littering hiking trails and a parking lot with rocky debris.
Geologists like Mr. Pierce note that earthquakes and geysers are life-giving forces in Yellowstone. Super-heated pools and steam vents are all intricately connected by a complex underground system whose main fuel is volcanic magma a few miles beneath the surface.
The liquid rock heats pockets of subterranean water and sends it racing toward the surface in the form of boiling pools and steam.
In turn, temblors jostle the natural wonders, rechanneling the flow of water, which results in some geysers going to sleep and others being born.
The geysers reaffirm Yellowstone's status as a global geological "hot spot," paralleling the volcanic activity of the Pacific Rim's volatile "ring of fire."
The analogy often invoked is that Yellowstone is like a piece of wax paper being tugged slowly over an open flame. The changes beneath the surface, however, are so intricate, that sometimes only surprise events like Steamboat's eruption offer clues to what's happening in the earth's crust.
"Although the geyser is a pinprick compared to a volcano," Deutch says, "it offers us an insight into the power of this vast volcanic region."
(c) Copyright 2000. The Christian Science Publishing Society