If it were to blow, it would not be the first the lake has seen. An explosion at the northeast edge about 13,000 years ago left a three-mile-wide crater at Mary Bay. The larger West Thumb of the lake was the result of another blast.

Like Harry Potter

While scientists were scanning the lake with sonar equipment in September 2003, one long-time Yellowstone researcher noticed an especially strong sulfur scent rising from bubbles in the water. He'd spent years on the lake but never noticed the smell to be so strong.

But the observation came at a time when it was unusual to be on the lake. Researchers typically leave by summer's end. In the fall, the lake is nearing its lowest levels, when there's less mountain runoff to dilute the sulfur-tainted water from underground hydrothermal venting.

"Maybe it's been that way during that season every year for a long time," Morgan said. "We don't know."

Meantime, there was a shift this year in the baffling water table at the Norris geyser basin about 20 miles away — leaving some former bubbling areas dry and creating neon green pools elsewhere that can scald to death wayward bison.

With at least one long-dormant geyser spitting to life near a trail, the park was forced to shut off a large portion of the boardwalk that winds through the steamy plateaus.

"Safety first," said Heasler, the park geologist. "The problem is, we don't know what's causing this."

To children, he compares the enigma of Yellowstone geology to the seven volumes that are expected in the Harry Potter series.

"It's as if we're just into the first paragraph," he said. "There's an awful lot we don't know yet."

He emphasizes that discoveries such as the spires and the bulge are newly noticed, not necessarily new. So Heasler said they couldn't be taken as evidence that there had been any radical developments at Yellowstone in recent years.

The shift at the Norris geysers, he said, is the same sort of change that has made the place remarkable since scientists started paying attention. It would be more unusual if things stopped changing.

Still, Heasler said he received several anxious e-mails a week from people worried about an eruption at Yellowstone that could kill millions.

Bob Smith, a geophysics professor at the University of Utah, has been studying what he calls the living caldera" of Yellowstone for decades. He noted that there have been no unusual seismic activities at the park this year that might precede bigger trouble.

"These things don't go like clockwork," said Smith, author of Windows into the Earth: The Geologic Story of Yellowstone and Grand Teton National Parks. "The hazard . . . is almost too small to calculate."