Such catastrophic possibilities excite audiences because they contain a particle of reality - they all are not only theoretically possible, but also historically documented. At one time or another, they have befallen the planet.

Yet another blockbuster is right around the corner - geologically speaking - and it threatens the whole western half of the United States. It is a massive eruption of the vast caldera located in Yellowstone National Park.

Many, if not most, of the 3 million tourists who visit Yellowstone annually may not be aware that as they watch the marvels of the Old Faithful geyser, they are sitting in the cone of an active volcano that erupts with nearly the same reliability - albeit with a longer period of dormancy, but a vastly increased volume.

Yellowstone erupts every 700,000 years or so. When it does, it sends more than 600 cubic miles (1,000 cubic kilometers) of debris into the atmosphere.

By way of comparison, when Mount St. Helens erupted on May 18, 1980 - the most destructive volcanic event in U.S. recorded history - it killed 57 people, destroyed 200 homes, 157 miles of roadways and 15 miles of railroad track, and wreaked a total of about $1.1 billion in economic damage.

That eruption ejected about 1.2 cubic miles (2 cubic kilometers) of debris.

Yellowstone's biggest event occurred 2 million years ago, ejecting 1,500 cubic miles (2,500 cubic kilometers) of rock into the air. Waist-high layers of volcanic ash or tuff from that eruption have been found in Iowa, nearly a thousand miles away.

Here is the bad news: the last eruption occurred about 700,000 years ago - and the two before that likewise were separated by 700,000 years.

"The current thinking has not changed for several years," said John Valley, professor of geology at the University of Wisconsin-Madison. "There has historically been a clear recurrence interval at the Yellowstone hotspot."

It is not unreasonable to think another gigantic eruption may happen within the next 100,000 years, and perhaps sooner, he said.

"That's the blink of an eye geologically, but it's not going affect anyone's decision to buy a house," Valley told UPI's Blue Planet.

Perhaps as time passes, these eruptions get less intense? "No," he said.

Based on past experience, the consequences to humanity from such an eruption could be serious in the extreme.

About 70,000 years ago, the Toba eruption on the island of Sumatra in Indonesia ejected about 1,700 cubic miles (2,800 cubic kilometers) of rock and ash into the atmosphere. Anthropologist Stanley Ambrose, of the University of Illinois at Urbana-Champaign, said this event alone came very close to wiping out humanity.

All that rock and dust kicked into the atmosphere initiated a six-year winter, followed by a 1,000-year ice age, he said. This resulted in a dramatic reduction in human populations across the globe - a population bottleneck.

One estimate is that 70,000 years ago - right after the Toba explosion - the number of female humans on Earth was reduced to between 4,200 and 4,600.

"Every geneticist who has done an analysis comes to the conclusion that we're missing most of the genetic diversity we expect for a species of this age," Ambrose told Blue Planet.

He said ice-core evidence - which can be delineated on an annual basis - shows a dramatic drop in global temperatures for six years after the Toba eruption - averaging 27 degrees to 29 degrees Fahrenheit (15 degrees to 16 degrees Celsius). Continuing cold temperatures show up in the ice core for another 1,500 years, he added.

Many of the interesting things happening at Yellowstone are going on underground. The park sits atop what scientists call a geologic hot spot, essentially a bubble of magma, close to the surface.

Hot spots sit still within Earth's mantle, while the continental plates drift across them. The best-known example of this phenomenon is the Hawaiian island chain, which has inched toward the northwest aboard the Pacific plate, while the hot spot's periodic eruptions have created the islands one at a time for millions of years.

At Yellowstone, the hot spot has created the Snake River Plain's fanning out to the west from the park.

Though an eruption of the Yellowstone caldera is virtually inevitable at some point in the distant future, it probably will not be soon - but just to be on the safe side, the National Park Service and the U.S. Geological Survey have established the Yellowstone Volcano Observatory.

The good news is things are slow right now. During February 2005, the YVO reported, there were only 61 earthquakes in the region, the largest registering only a 2.8 magnitude on the Richter scale, not even strong enough to be felt on the surface.

"These sorts of things have been going on since people started going to Yellowstone," YVO scientist in charge Jake Lowenstern told Blue Planet.

"It's a very active geological setting and has been that way for some time."

The ground in the caldera "breathes" regularly, rising and falling in response to the activity below. Between the 1920s and the 1980s, the ground rose about three feet, but since has subsided. Recently, the surface in one area of the caldera rose about six inches (15 centimeters), but that, too, has subsided.

"We know from studies of lake terraces that during the last post-glacial period - from 15,000 years ago to roughly the present - there has been as much as 10 feet of upward movement," Lowenstern said. "So what we've seen recently is smaller than that."

Valley said a large uplift would be one indication of a possible eruption. Other signs would include carbon-dioxide releases with carbon and helium isotope ratios indicating the gas originated in the mantle. The rising magma also would release sulfur and chlorine gases.

Geologists have gotten very good at assessing these changes, using them to warn of pending eruptions, such as the recent ones at Mount St. Helens and at Pinatubo in the Philippines.

An eruption need not be catastrophic, either. Yellowstone's last eruption released a large lava flow that created the 130-square-mile Pitchdome Plateau. It was a slow-moving lava eruption that had only local consequences, Lowenstern said, not national or global ones.

Blue Planet is a weekly series examining the relationship between humans and the environment, by environmental reporter Dan Whipple.

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