"This is a true story. It just hasn't happened yet."

That's part of the marketing promotion for a two-hour TV "docudrama" airing Sunday on The Discovery Channel, and it has grabbed the attention of Wyoming scientists, tourism officials and many others.

The fictional plot of "SuperVolcano" - produced by BBC Science and The Discovery Channel - revolves around a series of violent eruptions at Yellowstone National Park that send vast amounts of rock, gas and ash over three-quarters of the United States. It causes a "volcanic winter," devastating global agriculture and resulting in mass starvation.

The program is based on extensive scientific research about a real-life "supervolcano" under Yellowstone that has erupted before - 2.1 million years ago, 1.2 million years ago and 640,000 years ago. The three eruptions, respectively, were about 6,000, 700 and 2,500 times larger than the May 18, 1980 eruption of Mount St. Helens in Washington state.

Some geologists say another big eruption is overdue. Other scientists say chances are highly unlikely for such an event anytime soon.

"The show tries to envision what would happen if we had another super-eruption like we had 2.1 million years ago," said Jake Lowenstern, director of the Yellowstone Volcano Observatory.

The producers, writers and scientists behind the show talked to Lowenstern and many other scientists who study Yellowstone's geological features. In the show, actors portray leading scientists observing and reacting to the eruption.

"I have to say that in all the important respects, they got the details right," Lowenstern said.

According to the scientific speculation in the docudrama, a super-eruption would:

* Block highways as millions flee the unfolding disaster.

* Kill hundreds of thousands as the ash swamps towns and cities.

* Devastate America's food-producing regions.

* Knock out modern communications.

* Form sulfuric acid droplets in the atmosphere, blocking sunlight and causing global temperatures to plummet and devastate crops.

That being said, Lowenstern emphasized that a super-eruption beneath Yellowstone "is not likely to occur anytime soon, or maybe ever."

He readily acknowledged that all previous supervolcano eruptions around the world took place well beyond historical experience, so geologists have had zero experience studying how a supervolcano gears up for a super-eruption.

"We are getting pretty good at forecasting eruptions when volcanoes are restless," he said. "So far, there is no sign of that with Yellowstone … My recommendation is to relax and take this program as an educational opportunity."

Key science

The underlying science for the docudrama and documentary is concentrated in a research paper published by the Geological Society in the United Kingdom. That paper warns that the effects of a super-eruption beneath Yellowstone would "be sufficiently severe to threaten the fabric of civilization" - outdoing other natural disasters such as earthquakes, the Asian tsunami or historic volcano eruptions.

"We don't want to be sensationalist about this, but it's going to happen," said Professor Stephen Self, a geologist at the Open University in Milton Keynes and a member of the working group that produced the report. "We just can't say exactly when. But we have just had a natural disaster (the Asian tsunami) affecting hundreds of thousands of people. Now is the time to be thinking about this."

On average, supervolcanoes blow about every 100,000 years, scientists say. A "recent" super-eruption struck at Toba in Sumatra 74,000 years ago. Some scientists believe that super-eruption drove the human race to the edge of extinction. DNA researchers have found much less genetic variability in the human race than expected, given how long modern humans have been around. That suggests human numbers could have dropped to about 10,000, probably as a result of the effects of climate change driven by the Toba super-eruption.

Members of the Geological Society working group note the largest volcanic eruptions in recent centuries - Tambora (1815), Krakatoa (1883) and Pinatubo (1991) - have disturbed global climate for two to three years after the eruptions. Yet super-eruptions are up to hundreds of times larger than these, and their global effects are likely to be much more severe, resulting in the devastation of world agriculture, severe disruption of food supplies, and mass starvation.

Plan for disaster

Professor Stephen Sparks of Bristol University, a co-author of the new report, said civil contingency plans would need to be similar to those for a nuclear war.

The Yellowstone Volcano Observatory, founded in 2001, is working on a full hazard assessment about volcanism at Yellowstone, with a draft report due in 2006.

Jim Case, planning director for the Wyoming Homeland Security Office, said the state has a flexible emergency management plan already in place which can quickly be fine-tuned to specific natural disasters such as forest fires, earthquakes, floods and blizzards.

The state's plan can also be adjusted to deal with volcanism in Yellowstone, Case said.

Officials at the Yellowstone Volcano Observatory believes the buildup preceding a super-eruption would be detectable for weeks and perhaps months to years.

Precursors to volcanic eruptions include strong earthquake swarms and rapid ground deformation and typically take place days to weeks before an actual eruption.

According to the observatory Web site, about 80 relatively nonexplosive eruptions have occurred since the last super-eruption 640,000 years ago. The most recent volcanic eruption at Yellowstone, a lava flow on the Pitchstone Plateau, occurred 70,000 years ago.

Yet Ailsa Orr, producer of "SuperVolcano," said that when the program team presented the super-eruption scenario to the U.S. Federal Emergency Management Agency, the agency admitted it had given little thought to such an event happening on American soil.

About the show

"SuperVolcano" is scheduled to air at 6 and 9 p.m. MDT Sunday on The Discovery Channel, followed by two half-hour documentaries, "Supervolcano: The Truth About Yellowstone." The programs are also scheduled to air April 16 and 23.

To learn more

* The Yellowstone Volcano Observatory Web site is http://volcanoes.usgs.gov/yvo/.

* The Discovery Channel's Web site is http://dsc.discovery.com.

* The underlying science for the docudrama and documentary is concentrated in a research paper, www.geolsoc.org.uk/template.cfm?name=Super2, published by the Geological Society in the United Kingdom.

Two types of eruptions

* Normal volcanoes are formed by a column of magma, molten rock, rising from deep within the Earth, erupting on the surface and hardening in layers down the sides. This forms the familiar dome or cone-shaped mountains. There are thousands of these normal volcanoes throughout the world. Around 50 erupt every year.

* Supervolcanoes are different in almost every way. The term "supervolcano" implies an eruption of magnitude 8 on the Volcano Explosivity Index, meaning that more than 250 cubic miles of magma (partially molten rock) are erupted.

Sources: BBC/Discovery Channel and the Yellowstone Volcano Observatory