YELLOWSTONE NATIONAL PARK — When European settlers wandered upon this otherworld of gurgling mud pits and angry geysers, they described it as a place where "hell bubbled up."

They didn't guess, as geologists believe now, that three times in the past 2 million or so years, hell blasted the Earth's crust here with a fury that can barely be imagined.

Most recently, some 640,000 years ago, Yellowstone's rage toppled mountainsides, changed the course of rivers and sprayed ash ankle deep over all of what is now the Western United States.

So there's understandable interest in whether it might blow again. And when.

Fresh high-technology studies of the underground cauldron — and discovery of a bulge on the floor of Yellowstone Lake — show anew the region as geology-in-the-making.

There's evidence that the bulge — described by one scientist as an "inflated plain" — might be throbbing from the pressure that pushed it up in the first place.

That detection has scientists captivated, not frightened, even as it fills amateur geologists with dread.

Those laymen worry that the pressure cooker of Yellowstone is set to burst.

Even smaller blasts — say the size of Mount St. Helens — that come about every 20,000 years or so can rearrange Yellowstone's scenery. The most recent of those was 70,000 years ago.

Some urge government engineers to gradually vent steam and magma by drilling, rather than wait for a seemingly imminent, giant and calamitous blast.

"If nothing is done there will be an unimaginable disaster," went discussion at one Internet discussion site. But nobody even seems to be thinking about it."

But the geologists who explore the caldera — the collapsed supervolcano that is Yellowstone — share neither such alarmist doom nor faith in methods for taming the forces boiling underground.

For starters, drilling here would spoil the natural setting of the world's first national park, created in 1872, said park geologist Hank Heasler.

What's more, he said, it would do no good. The magma chamber miles below the park is mostly like a hardened sponge and is essentially self-sealing.

"Besides, it's too big," he said, noting the caldera measures 35 miles by 45 miles. "We're on the skin of the apple. We can leave little bruises, but we can't affect the flavor of the fruit."

Discovery of the bulge

Government and university scientists dismiss new-born worries about Yellowstone, about the bulge beneath the lake, and about recent changes at the park's Norris geyser basin. Mostly, they marvel at their out-sized laboratory.