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 others 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.

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 massive 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."

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.

"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 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."

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.

While park researchers try to put their findings in perspective, others have opted for doomsday scenarios.

One Web site urged everyone within 600 miles of Yellowstone to ready for a major eruption while accusing 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."

The state of Wyoming is trying to defuse unwarranted fears through education.

"There is a degree of fear generated that is largely unfounded," said Jim Case, who heads the geologic hazards section of the Wyoming State Geological Survey. "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."