Far below the blue waters of Yellowstone Lake, a mysterious dome 2,100 feet across and 100 feet high is causing concern among scientists and citizens who don't know whether it's a harmless curiosity or a hazard on the verge of exploding.
The dome, also called a bulge or an elevated plain, is less than a mile from shore and was recently explored by researchers with the U.S. Geological Survey, using unmanned submarines and sonar.
"It could be the precursor to a hydrothermal explosion," said Lisa Morgan, a geologist leading the team. "It's a pretty significant feature."
Hydrothermal blasts occur when super-heated water, often under extreme pressure, rapidly flashes to steam, hurling rocks and sometimes gouging out huge craters.
News of the dome comes at a time of increased activity beneath Yellowstone, which experienced a magnitude 4.4 earthquake in August.
In July, the park shut down part of a popular trail near the Norris Geyser Basin because the ground heated up to 200 degrees. Steamboat Geyser, the world's tallest, has sometimes gone 50 years between eruptions but has spouted three times this year.
The events have sparked Internet chatter and fear from some that a catastrophe is at hand. But scientists and park officials have cautioned against panic, saying there is no evidence of any immediate threat and that Yellowstone is intensely monitored for any changes.
Morgan said the dome could have been in the lake for up to 10,000 years. And while it may explode, it might just as easily collapse or simply do nothing.
Still, she and park officials are drawing up a hazard-assessment plan just in case.
"A hydrothermal explosion is an extreme event and a rare event, but they have happened," Morgan said.
Mary Bay, an area of the lake near the dome, was created by a hydrothermal blast more than 13,000 years ago that scientists consider to be one of the biggest explosions in geologic history. There are at least five other craters in Yellowstone Lake that were created by massive eruptions.
Exactly what damage an explosion would cause today is being investigated. Morgan said it could eject rocks and poisonous gas and cause 20-foot waves. Whether damage would spread beyond the park would depend on the force of the blast.
"There are a lot of scenarios we are trying to put together," park geologist Hank Heasler said. "No one has ever witnessed a large hydrothermal explosion. It's a steam explosion, which can be as powerful as TNT."
Heasler keeps tabs on the rising temperatures in the park through a network of sensors.
Plumes of steam and gurgling, belching caldrons of hot water dot the surreal landscape where the trail was closed. Most of it remains closed to the public.
"The message is that Yellowstone is dynamic," Heasler said. "It changes daily."
Park Sits Atop Volcano
The park, which gets 3 million visitors a year, sits atop one of the most active volcanoes on Earth, a deep caldera 45 miles long and 30 miles across, with more than 10,000 vents, geysers and bubbling pools of hot water.
Scientists compare the place to a huge set of lungs that rise and fall. Others say it's like a piecrust expanding with steam and then subsiding as heat escapes through the crust.
"Yellowstone is a living, breathing caldera," said Bob Smith, professor of geophysics at the University of Utah and a coordinating scientist at the Yellowstone Volcano Observatory. "We can see the ground rising and falling in Yellowstone. It's a complex place with a lot of thermal energy."
Geologists estimate that the last big eruption happened 640,000 years ago, when the volcano sent ash as far south as Texas. Scientists say it was 1,000 times more powerful than the Mt. St. Helens cataclysm in 1980. There have been 30 eruptions since.
Jake Lowenstern, scientist in charge at the Yellowstone Volcano Observatory, said predicting such things was difficult.
"We don't have an easy way of knowing what features will become unstable," he said. "Small explosions happen every year, large ones every few hundred years."
When it's not erupting, the ground beneath the pine forests and bison herds routinely shakes and slides. Last year, there were 2,375 earthquakes. Most are small, but a temblor registering 7.5 on the Richter scale killed 28 people just outside the park in 1959.
One of the liveliest areas lies beneath Yellowstone Lake.
Flanked by geysers and roiling turquoise pools, the lake has more than 110 miles of shoreline and sits at 7,733 feet, making it the largest high-altitude lake in North America. At its deepest, the lake is at least 320 feet deep; it has an average depth of 140 feet.
The floor of the deep-blue lake, whose bays and lagoons were created by past eruptions, remained largely a mystery until researchers using cameras on robot submarines began exploring it.
"This is the moon, underwater," Morgan said.
Down among the shadows, the team saw more than 250 thermal vents, fissures, geyser basins and columns of silica soaring 30 feet high.
"Everyone is paying attention to the bulge, but it's only one feature," said Morgan, who has studied the lake for five years. "We found extensive fault systems and landslides. When you think of all the seismic energy that could be released, it could be extreme."
Morgan discovered the dome in 1999, but has recently been examining it more closely. Using sonar to scan the bottom of the lake, she has produced the most extensive mapping of the area to date, including close-ups of the dome. Morgan hopes to see if it's changing size and shape.
One fear is that a sudden drop in the water level could trigger an explosion as pressure is quickly removed from the dome, which sits 60 feet below the surface. An earthquake, heavy storm or landslide could accomplish this.
While Morgan and other researchers try to put their findings in perspective, others have opted for doomsday scenarios.
One Web site urges everyone within 600 miles of Yellowstone to prepare for a major eruption, and accuses the park of covering up the extent of the danger.
Another claims Yellowstone "will blow its cork" in the next six months, causing "the three days of darkness spoken of in the Bible."
Marshall Masters, who runs the futuristic Web site yowusa.com, claims that the passing of a planet called Nibiru through the solar system could be causing Yellowstone's restlessness.
"We are telling people to look for creatures that are not politically motivated," he said. "If the bison and deer start hoofing it out of the park, then it's time to throw the kids in the station wagon and head for Grandma's."
Such talk confounds park officials and scientists.
"We say the park is safe," said Yellowstone spokeswoman Cheryl Matthews. "The changes at Norris and the lake are part of the dynamics of the park."
The state of Wyoming is trying to defuse what it calls unwarranted fears through education.
"We talk to people who are concerned the park is going to have a massive volcanic explosion," said Jim Case, who heads the geologic hazards section of the Wyoming State Geological Survey. "There is a rumor the park blows every 600,000 years."
Case, who gives talks around the state about real and imagined hazards at Yellowstone, gets calls asking if it's safe to buy a house in Cody or Jackson, towns near the park.
"There is a degree of fear generated that is largely unfounded," Case said. "If we see changes in the seismic signatures, we will get the word out to the public. It's not to anyone's advantage to hide it."
Whatever happens, Morgan hopes to return to Yellowstone Lake next summer and take core samples of the dome. She also plans to build a laboratory model of it and simulate an explosion to see the effects.
Having worked at active geological sites in Japan, Indonesia, Central America and at Mt. St. Helens, Morgan said the discoveries in Yellowstone Lake stood alone.
"It's been the experience of my life," she said, sitting among color maps of the lake floor in her Boulder, Colo., home. "Here the Earth is forming in front of your eyes. Rarely do we get the opportunity to see such a thing."