The film uses specially commissioned research, studies of previous supervolcanic eruptions and predictions drawn from a range of scientific disciplines to tell the story of how such an eruption would affect the United States and the world.

Set in the near future, Supervolcano draws on scientific evidence available from a number of previous eruptions at Yellowstone as well as research, much of it specially commissioned for this production, from major scientific bodies, including the United States Geological Survey, Yellowstone Volcano Observatory, the Federal Emergency Management Agency, the UK Met Office and NOAA (The National Oceanic and Atmospheric Administration).

The characters in Supervolcano are based on the people who work for these organisations.

Supervolcanoes are on a totally different scale to ordinary volcanoes: Mount St Helens released enough magma to bury London beneath an ash layer a metre thick. A supervolcano would release enough to cover the entire UK under four metres of ash.

When Vesuvius erupted, Pompeii and at least 5,000 of its citizens were killed by a wave of red hot ash and gas called a pyroclastic flow. Flows from a supervolcano would scorch and destroy all life around the volcano in an area the size of an English county or small American state.

There are several supervolcano sites around the world but the one with the most lethal potential lies beneath Yellowstone. Satellites have photographed the mouth of the volcano and, at 85 kilometres long and 45 kilometress wide, it would easily swallow Tokyo, the largest city in the world. The chamber itself, five miles underground, is big enough to hold over 25,000 cubic kilometres of molten rock.

The land around Yellowstone regularly swells and subsides in response to the shifting levels of magma and volcanologists believe that one day the molten magma contained within the chamber will burst out - as it has done many times before. They just don't know when.

Accompanying the drama will be two half-hour documentaries, The Truth About Yellowstone, which will be shown on BBC Two immediately after the film. The documentaries examine recent volcanic activity at Yellowstone and give a range of views from scientists on whether Yellowstone is likely to erupt again in our lifetime.

The making of Supervolcano

Supervolcano took two years to be brought to the screen. In the process of making the film, major scientific bodies consulted by the production were prompted to consider their response to a possible super-eruption for the first time.

Here, the series producer, Ailsa Orr, explains how the idea for the drama first came about and tells the story of how the project was finally brought to fruition:

"The idea originated with a Horizon documentary produced in 2000," says Ailsa. "The reaction to that programme in this country, and of course the States, was amazing and Michael Mosley, the executive producer on Supervolcano, had the idea to dramatise the story. We thought a dramatisation would help people get their head around it because some of the facts are so extraordinary and visuals so dramatic that you really have to see it to believe it."

From the outset the major priority for the drama was that it was based entirely on scientific data to ensure the scenario was factually accurate. Therefore the opinions and advice of key scientists were sought from the start, and the decision was taken to base the eruption in the film on a real eruption which happened millions of years ago at the park.

"We started by examining data from the first super-eruption at Yellowstone which happened 2.1 million years ago," Ailsa says. "We also looked at the evidence of the last supervolcanic eruption on the planet which happened at Toba in Indonesia, 74,000 years ago. As far as scientific consultants went, the first port of call was Bill McGuire. Bill is a leading expert in the field of natural disasters. There is limited published material available on this phenomena and Bill helped us understand what supervolcanoes are. It's a relatively new science that people are only just starting to talk about and understand."

Bill came on board as series consultant.

It was equally important that the production spoke to the people who would be directly responsible for handling the catastrophe if it were to happen.

"A critical early step was going out to Yellowstone and meeting the scientists who run the Yellowstone Volcano Observatory (YVO)," agrees Ailsa.

"The three key people involved in running the YVO became the basis for the film's three central characters.

"Rick our protagonist is based on Dr Jake Lowenstein, the main scientist at Yellowstone; Bob Smith who oversees the Yellowstone operation from the University of Utah became Jock, our British character; and Hank Heasler, who is based at the park itself, became our character Matt."

The input of the YVO scientists was critical when it came to understanding how the scientists in the drama should handle the threat of a super-eruption. But were the scientists at Yellowstone a little worried about panicking their visitors by showing the effect of an eruption?

"They were incredibly helpful," says Ailsa. "Yes they were a little nervous about inadvertently causing unnecessary alarm but they were absolutely dedicated to helping us get it right."

Based on the dramatic scenario, a team from the University of Utah along with the UK Met Office were able to give their estimates of the size of the area which would be affected by volcanic ash produced by the eruption.

"From this, we created an 'ash projection map' which took into account wind direction and time of year of our eruption. Every time we refined our storyline we would send it back to them for approval so they were very closely involved and had to be," explains Ailsa.

Once the experts had provided their projections, Ailsa and her team talked to the Federal Emergency Management Agency (FEMA) who have responsibility for handling national disasters in the United States.

"FEMA had no contingency plans for a disaster on this scale; the largest disaster they've ever had to deal with was 9/11 and that stretched their resources to the limit. Our scenario would affect an area 10 million times greater than 9/11 did.FEMA were extremely interested in working with us to come up with a theoretical contingency plan as to how they might deal with it.They gave us data on how many people would be affected by the eruption in the US."

But it wouldn't only be people in the United States who would suffer from the consequences of a supervolcanic eruption - the effects would have an impact on the world.

The UK Met Office were instrumental in helping the team to understand how climate change would unfold following a super-eruption.

"One of the main gases ejected in an explosive eruption is sulphur dioxide which forms sulphuric acid when it gets into the stratosphere," says Ailsa."From there, it begins to spread around the world. The Max Planck Institute in Hamburg helped to model the spread of the sulphuric acid around the globe.

"We're talking about catastrophic amounts of sulphuric acid circling the world within just a few weeks. It forms a veil that blocks out sunlight, causing temperatures to plummet. he Met Office models predicted a drop of about 15 degrees across Europe and 20 degrees in the southern hemisphere, the monsoon would stop, crops would fail and somewhere in the region of one billion people would die through climate change and starvation."

Once the scientific projections had been obtained and the production team had a good idea of how a supervolcanic eruption would unfold, work could start on the script.

"We chose Eddie Canfor-Dumas because he had written Pompeii - The Last Day for us previously, and we knew he would construct a story that stayed faithful to the facts," says Ailsa, again emphasising the importance of factual accuracy.

The scripts, written by Eddie, were approved at every stage by all the scientific bodies consulted: the Yellowstone Volcano Observatory, FEMA, the Met Office and even the US Geological Survey who oversee the observatory.

With the writing underway, work began on the all important visual aspects of how to create the impact of an eruption on screen. Once again the scientists were the key to this process.

"The first thing we had to get right was to understand the dynamics of a supervolanic eruption - how it would unfold, what it would look like," says Ailsa. "It's very difficult to know for sure because nobody has ever seen a super-eruption happen but we consulted with a lot of scientists and the consensus of opinion was that a super-eruption is not just one big massive eruption but a series of separate eruptions that occur around the rim of the caldera (crater). Only towards the end of the eruption process do they all converge into one. Once this scenario had been signed off by the scientists, we got a storyboard artist to visualise it so everyone was clear on what we had to create in the film."

Having got to this stage, Ailsa and the team were then in a position to talk to a visual effects company:

"We decided to go with a visual effects (VFX) company called Lola, who we worked with on Pompeii - The Last Day. We really liked the method they suggested to recreate the eruption process. While other VFX companies wanted to use particle-based systems (essentially all the effects would be computer generated), Lola wanted to create the column and pyroclastic flows by using real, live elements. Their special effects team constructed a massive 'cloud tank' - a transparent water tank into which bleach could be injected under real pressure, filmed at high speed. The result was eruption columns and clouds that looked and moved like the real thing. Then they added the other CGI elements onto it."

Another strong 'character' in the film is Virgil - the virtual geophysical imaging laboratory, a holographic, 3D model of the park.

"Virgil was a clever idea that the director Tony Mitchell came up with," says Ailsa. "His first question when he came onto the project and was looking at photos of Yellowstone was, 'But where is it? Where is the volcano?' We had to explain to him that supervolcanoes exist beneath the ground, that they're almost impossible to spot. He wanted a way to show the viewers where the volcano is in relation to Yellowstone, and Virgil was the answer to that - it became a character in the film. Virgil doesn't actually exist but it could some day - it's our one nod to the future in the film."

Virgil was an instant hit with the scientists advising on the project.

"They all said they would love to have a tool like Virgil to bring all the information about the park together into one model. It would really allow them to see how all the small things that happen all over the park are related, and to what overall effect. They really think Virgil is great!"

Casting was the next critical part of the process.

"We deliberately went for actors who weren't particularly well known," Ailsa says. "Gary Lewis who plays Jock is probably the best known actor in this country, although he does mainly features and not a great deal of TV work. The rest of the cast are Canadian - we knew we were going to be filming there so it made a lot of sense to cast there. And we were really lucky - our brilliant casting agent and the director Mitch found us the cream of Canadian talent, including Michael Riley, who plays Rick our central character."

The filming took place in Vancouver. Ailsa explains:

All the filming was completed before the tragedy of the Asian tsunami on Boxing Day but what did the actors, who were almost all from the North American continent, make of the fact that the volcano may erupt at any time?

Ailsa says they were initially sceptical.

"At first none of them believed it. They assumed that Supervolcano was a Movie Of The Week, a very low budget TV feature that is a big part of American and Canadian TV culture. We worked very hard to convince them that it wasn't and to introduce them to the concept of factual drama which isn't known over there. After a while they all got very interested in it. They'd go home after work and look up the Yellowstone Volcano Observatory on the internet and come in the next day saying 'It has erupted before! The script is right!' Michael Riley, who plays our main character Rick, got really into it. He used to call up Jake Lowenstein - the scientist his character is based on - in his breaks and ask his advice about the scene he was about to do. After the call he'd amend the script based on what Jake had said to him and then I'd be listening to the scene thinking, 'I've never heard those words before'. Michael would say, 'Oh Jake gave them to me, don't worry about it…'

"I'd have to go and check them out anyway but I was very impressed with how much he took the role to heart and the great relationship he struck with Jake. He really got to grips with his character and the pressure he would face in a situation like this."

The budget was another factor that had to be grappled with in Vancouver, especially as many of the Canadians employed were used to working on Hollywood feature films.

"But we did it - and at the end the construction manager told us, 'I never thought you could do it. For the first four weeks I thought you were going to fire me because I kept saying it can't be done. Then I thought I'd just walk and then I thought, well I'll just stick around and see how it goes. And I must say I'm really impressed because you've actually done it!'."

For Ailsa, all the blood, sweat and tears were worth it when she saw the finished film.

"The big defining moment for me was a shot I only saw at the end - it comes half way through the second part. As a producer you're approving shot after shot and I thought it looked great. But one shot they saved to show me at the end. The first column on the volcano erupts and then the camera pulls back and you see another column erupt and then another and another and another and it's like Dante's inferno. Fifteen eruption columns and it just looks astonishing. That's when you realise what a super-eruption is and until that moment, I don't think you really can."

Fascinating facts

• A supervolcanic eruption occurs somewhere in the world approximately every 50,000 years or so.
• The last one happened in Indonesia 74,000 years ago - suggesting the planet might be overdue the next one….
• There are about 40 known supervolcanoes around the world, most are extinct.
• The one with the most lethal potential, however, is the Yellowstone supervolcano as it's situated in the heart of a country which is home to 300 million people.
• There are ancient calderas (supervolcanoes) in Britain: Glencoe in Scotland and the Peak District.
• Like most supervolcanoes, however, they are long-extinct.
• Yellowstone has experienced three supervolcanic eruptions in the ancient past: 2.1 million years ago, 1.3 million years ago and 640,000 years ago.
• Disturbingly, this cycle of approximately 600-700,000 years suggests another eruption could be brewing.
• The force of a supervolcanic eruption at Yellowstone would be the equivalent of 1,000 Hiroshima bombs exploding every second.
• The first super-eruption at Yellowstone ejected 2,500 times more volcanic material than the Mount St Helens eruption of 1980 - the most violent eruption in American history.
• Ash would have fallen across the whole of the United States.
• The supervolcanic crater at Yellowstone is over 80 kilometres long by 45 kilometres wide - large enough to hold the world's biggest city, Tokyo (population 18 million).
• A super-eruption at Yellowstone would eject over 2,000 million tonnes of sulphuric acid into the stratosphere.
• This would form a veil around the earth that deflects sunlight, triggering a catastrophic volcanic winter.
• Temperatures would drop by up to 12 degrees in the northern hemisphere and up to 16 degrees in the southern hemisphere causing mass starvation as the life-giving monsoon rains would fail.
• The total worldwide death toll from a Yellowstone super-eruption has been estimated at 1 billion.
• Today, the ground at Yellowstone emits 30 to 40 times more heat than the average for North America.
• That heat powers Yellowstone's hydrothermal attractions - the world's greatest display of geysers, hot springs, mud pots and steam vents.
• The heat comes from the vast body of magma that resides only a few kilometres beneath the ground.
• On average at Yellowstone, one square metre of ground emits an amount of heat equal to two watts. So if the heat released from 50 square metres of ground was converted to electricity, it would light a 100W light bulb.
• Yellowstone as a whole emits about 5GW, enough to power a city of more than 2 million people.

Stephen Hunt