YELLOWSTONE NATIONAL PARK, Wyo. - 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 people 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 caused by big eruptions.
Exactly what damage an explosion would cause today is being investigated. Morgan said it could eject rocks and poisonous gas and cause waves as high as 20 feet. Whether the damage would spread beyond the park depends 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 cauldrons of hot water dot the surreal landscape where the trail was closed. Most remain shut to the public.
"The message is that Yellowstone is dynamic," Heasler said. "It changes daily."
The park, which draws 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 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 Mount Saint Helens cataclysm in 1980. Thirty eruptions have occurred since.
When it's not erupting, the ground beneath the pine forests and bison herds routinely shakes and slides. Last year, 2,375 earthquakes occurred. Most are small, but a temblor registering 7.5 on the Richter scale killed 28 people 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 and has an average depth of 140 feet.
Deep blue, with bays and lagoons created by past eruptions, the lake floor remained largely a mystery until researchers using cameras on robot submarines began exploring it.
"This is the moon, under water," 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."