if the same volcanic acid cloud swept over europe today. using our model, we can create a kill map. what we find is tens of thousands of deaths in the uk, in the netherlands in germany, in france, all over central and western europe. if we calculate the total, we find a mortality rate of more than 100,000 people. this is quite an astonishing number. narrator: since her model shows that the cloud would eventually spread across the entire hemisphere it's a stark warning of what we might expect in american cities. but there's another threat from iceland's volcanoes. 45 miles west of laki is hekla.

at almost 5,000 feet hekla's distinctive cone towers above the landscape. she's one of iceland's most active volcanoes, erupting over 20 times in the last thousand years. hekla last erupted in 2000 and recently her eruptions have been mild. but thor's discovered a place that shows just how explosive she can be. and with hekla it's all about ash. the snow-covered mountain in the background there is the hekla volcano itself. and here in the foreground this light-colored layer there at the top was erupted from hekla 3,000 years ago. narrator: these ash layers tell thor a great deal

about hekla's explosive past. thordarson: the reason why we spend our time studying these ash layers is that we can get information about the nature of the eruption, its intensity, its explosive power, and how widely it affected the environment. narrator: he's discovered one particular eruption from hekla 3,000 years ago which threw out a tremendous amount of ash. if you look at the landscape around us, all the surface is covered with the ash, which is 15 feet thick. and it doesn't matter where you look, it's all around you. that gives you a good indication of how powerful, how strong that eruption was. narrator: hekla's explosive eruptions created this barren landscape. it's made of layer upon layer of ash, formed from pulverized magma.

and it's typical of hekla's family of volcanoes-- the stratovolcanoes. thordarson: most stratovolcanoes around the world will produce big, explosive eruptions. every so often we have a big bang. (explosion) man (on walkie-talkie): now we've got an eruption down here. narrator: it's exactly what happened in washington state in may 1980. mt. st. helens, america's most famous stratovolcano, erupts. rising magma triggers an earthquake and a vast landslide... man (on walkie-talkie): the whole northwest section blowed up. narrator: ...sending ash 80,000 feet high in just 15 minutes... this is far east as you can go. narrator: ...and causing over 50 deaths... man (on walkie-talkie): i can't believe this. doesn't look like any place i've ever been before.

narrator: europe's most famous stratovolcano is mt. vesuvius. when it erupted 2,000 years ago, gases and hot ashes rained down on the towns of pompeii and herculaneum, asphyxiating an unsuspecting population and killing up to 25,000 people. but what thor's discovered about hekla's eruption 3,000 years ago makes mt. st. helens, mt. vesuvius and eyja's 2010 eruption pale in significance. thordarson: we've learned that this is one of the biggest eruptions from hekla, in the last 10,000 years. and the total volume of this material is between two and three cubic miles. it also sent ash plumes extending from greenland well into mainland europe.

if we were standing exactly in this location 3,000 years ago when hekla erupted, the first thing we would have seen is a humongous column of ash rising up from the volcano. and from that plume, we have ash raining out. and along with that, you would have rocks like this one here flying through the air. this one is 20 pounds. so this eruption it would have killed you and you would have been buried in 15 feet of ash. narrator: when eyja erupted, her ash grounded aircraft for almost a week. but hekla could be far more disruptive. 3,000 years ago, hekla produced 75 times more ash. the concern is she might be about to do it again. thordarson: the pattern that we have detected is suggesting that right now it is at the end of a phase

with eruptions diminishing in size, indicating that we might be entering a new phase which usually starts with a big, explosive eruption. so it's possible that hekla might produce a very big, explosive eruption in the near future. narrator: what chaos might we expect if this happens? claire witham has been asking the same question. she's part of a rapid response team at the london volcanic ash advisory center at the uk's met office. it's the first line of defense against volcanic ash for europe's aviation industry. and when eyja erupted, it put the team through their paces. we hadn't seen anything that had really blown an ash cloud from iceland to our area in recent history, so it hadn't really got into people's minds

that that might happen. the eyjafjallajokull eruption went on for about 39 days. people were working around the clock here on a scale that we hadn't really experienced before. narrator: the minute an eruption occurs, observers in iceland contact the team. witham: we then have a race against time to get out an advisory. we then have to update that information every six hours until the end of the eruption. narrator: claire needs two key things to predict where the ash cloud is going. first, she needs an accurate weather forecast. witham: the weather's really vital because when the ash is erupted into the atmosphere, it then gets transported by the winds at those higher levels and gets taken wherever those winds are going. narrator: how forcefully the volcano erupted is also critical. witham: we need to know how high the eruption is and also how much has been emitted and how that changes with time. narrator: armed with this information in 2010, claire and her team used supercomputers

to model the movement of ash from eyja. so what we can see is the start of the eruption, and then the ash is transported to the southeast and is then dispersed across the northern hemisphere. narrator: in the future, with the possibility of bigger eruptions, the team's predictions will be even more important. to see what might happen nova asks claire to make a model using hekla's huge eruption 3,000 years ago, which belched up to three cubic miles of ash and the weather conditions when eyja erupted in 2010. so this simulation shows the first ever model run of a much larger eruption. and what we can see is that because this eruption was much bigger, so it went much higher in the atmosphere and produced considerably larger amounts of ash, the plume covers a much wider area, and the levels of ash in the plume

are much greater, which is shown by this red color, spreading out over a much larger region of europe and also across the northern hemisphere. and what this tells us is that for a future eruption of a larger size, the potential for disruption is greater. narrator: what's worse is that we might not have time to prepare. thordarson: hekla does not give us much warning. her temperament is such that she will flare up 30 minutes before it erupts, so she can surprise us. narrator: as scientists are learning iceland's home to a group of deadly volcanoes-- eyja blew in 2010; laki sent a toxic cloud of sulfur circling around the northern hemisphere; and hekla, which could unleash millions of tons of ash without warning. but the greatest threat is from their restless neighbor.

rampino: i would be more worried about a katla-style eruption happening today. narrator: one that volcanologists can't afford to ignore. the concern with katla is that it might erupt any time. narrator: the volcano hidden under the ice. katla's rumbling. narrator: some fear her eruption could be worse than laki's sulfur cloud and hekla's ash put together. katla erupts every 50 years on average. her last major outburst was nearly a hundred years ago so an eruption is long overdue. all the signs that we detect from the volcano are telling us that it is building up to an eruption. narrator: recently scientists have detected swarms of earthquakes around her crater.

whether this threatening activity is because eyja's eruption has lit katla's fuse no one knows. to find out what's going on, sigrún hreinsdóttir and freysteinn sigmundsson are making the three-hour trip to the volcano's summit to set up monitoring stations. hreinsdóttir: we are driving now up to the glacier and we need a guide because it's a fairly rough environment and there are crevasses to worry about. narrator: it's a treacherous journey. katla's crater is over six miles wide, buried below the ice. sigrún is looking for a specific site to set up one of her stations-- somewhere where katla's rim breaks through the ice. hreinsdóttir: right on the top of the bedrock there. it's actually... the bedrock is the caldera rim

sticking right out of the glacier. sigmundsson: what a fantastic site! narrator: it's the perfect spot. this rocky outcrop connects to katla's magma chamber deep below. if the magma moves down there, so should the ground above it, here. hreinsdóttir: this is just a simple antenna that is receiving data from the satellites that are travelling across the sky and we hook the antenna to a receiver and the receiver is logging the data. narrator: the satellites transmit signals to sigrún's gps network stationed around katla's rim providing an extremely accurate fix on their positions. if katla's magma chamber inflates its rim pushes outwards, moving the receivers. even a tiny shift of less than an eighth of an inch can be detected.

4-2-9-point-3. so magma, molten rock, is coming from great depth into the volcano and what we have is like a magma chamber, which is just like a balloon if you imagine you put air in a balloon and it inflates. we can actually sense that on the surface. narrator: and there are indications that katla's chamber is indeed inflating. hreinsdóttir: so freysteinn, we can actually look at the data from the gps station that is right here at katla volcano. and we set this up during the eyja eruption. this is 2010 and now we are in 2012 and you can see that the overall trend here, that is up, so inflation of the magma chamber is almost eight centimeters. it's quite a bit. the magma chamber of katla volcano is inflating. narrator: the gps readings show katla could be building to an eruption.

so what's the worst we can expect from her? thor's been investigating layers of black ash the volcano ejected in 934. what he's uncovering alarms him. so here we have the lower part of the deposit. here is the bottom, and it's fairly coarse grained. this is the one thing that we really want to worry about. explosive events like this produce a lot of ash. narrator: volcanologists have measured the thickness and extent of these ash layers, allowing them to calculate the volume of material katla erupted a thousand years ago. the results tell us this eruption of katla in terms of volume of material is double the hekla eruption 3,000 years ago.

narrator: the closest we've come to experiencing an eruption on that scale rocked the philippines in 1991 when mt. pinatubo erupted. this, one of the largest eruptions in modern history, rumbled for just over a week sending an estimated 2.5 cubic miles of ash into the atmosphere. but katla's ash reveals she can erupt for far longer. thordarson: there are many, many many layers there, which means that this eruption featured many, many explosive events. it's a complex eruption; it probably lasted for at least one year. narrator: if a yearlong eruption of ash wasn't threatening enough, katla also spewed 220 million tons of sulfur-- much more than laki in 1783. it is double that of laki.

so it is the biggest eruption in icelandic history and it is the biggest volcanic pollutant in recent times on planet earth. narrator: it's this acidic pollution that scientists fear the most. if katla erupts today, the devastation could trump the effects of eyja, laki and hekla combined. and the amount of sulfur produced would be far worse. well, sulfur has a sort of a double whammy effect when it's in the atmosphere. the little droplets of sulfuric acid or aerosol, as it's called, stay in the upper atmosphere and cut out some of the sunlight that would come in and warm the earth so the result is that the earth gets cooler. narrator: katla has the power to change the world's climate.

rampino: a long-duration eruption such as katla, if it happened today would cause quite a mess on a global scale. the numbers are about two degrees celsius cooler. it doesn't sound like much but it's enough to cause a transformation of the landscape. narrator: the evidence is clear. iceland's volcanic eruptions could have global consequences producing enough ash and sulfur to change the world's climate. but could we protect our economy if a less powerful eruption, like eyja in 2010, occurs in the near future? it's a vital question for the airline industry as aircraft will be the first to feel the effects.

at 35,000 feet, ash particles are so fine they are invisible to the naked eye. so when eyja erupted thousands of planes were immediately grounded. it's a problem aircraft engineer ian davies has been grappling with. davies: well, we know ash and engines don't mix, because this modern turbo fan engine behind me basically takes in hundreds of cubic meters of air every second. narrator: any volcanic ash sucked into the engine-- which heats up to 2,000 degrees fahrenheit-- melts and sticks to vital parts. and when it does that, it will cause a surge stall, which is effectively a backfire, and in doing so we can get flames out the front and out the back of the engine... and ultimately it will shut down.

narrator: fred prata, an imaging expert, is developing a unique system that could help. it's called avoid. prata: this instrument here is a fast sampling dual camera system measuring in the infrared. infrared radiation is heat. every molecule in the atmosphere that's vibrating emits infrared radiation. narrator: ash particles also emit infrared radiation at a particular wavelength. prata: now, they have a particular spectral signature in the infrared which is quite different to that of other things in the atmosphere like water clouds ice clouds, water vapor. narrator: the cameras, which can pick up infrared signatures, provide pilots with a picture of the sky up to 60 miles ahead, allowing them to distinguish between ash and clouds. prata: so now he can start to see where the ash is. if it is there and he sees it

five or six minutes ahead of time, he can then make a small deviation and avoid it completely, safely and economically and land his aircraft. narrator: the new system could be the first step towards keeping planes in the air when ash next rises from iceland. and it could help pilots map and maneuver around invisible ash clouds while they're flying, even reducing the need to ground planes. still, the greatest threat would come not from katla's ash, but from her toxic sulfuric acid cloud. faced with a giant volcanic eruption today, a major response would be needed to protect us.

grattan: but how would they do that? they'd do that by switching off the power stations that might be burning coal. they'd do that by stopping us driving cars that are emitting pollution from the engines. they'd do that by closing factories down. i think modern-day society should very much be concerned with volcanism. i think we're probably more susceptible to these types of events in the 21st century than we were even in the early 20th century. we think we're immune to these things and frankly we're not. narrator: those least able to protect themselves would be most affected. grattan: to be brutal about it, the vulnerable population will be culled from society: the elderly, the sick, people with heart and lung conditions, the obese. we are going to see many, many people dying. narrator: today we have a society dependent on air travel. davies: the way we move food freight, people around the world now any disruption to that is devastating.

narrator: a society that's delicately linked together. grattan: so a massive volcanic eruption is really going to disrupt this web of connectivity. all those networks are going to be fractured. they're not going to work anymore. narrator: lives were thrown into chaos when eyja a relatively small volcano awoke in 2010. how much worse will it be if and when one of iceland's doomsday volcanoes erupts again? neanderthals rediscovered. who were these primitive cavemen from our prehistoric past? man: neanderthals have the mother of all image problems. they're brooding they're stupid-looking they have no personality. new evidence paints a startling picture.

this is the smoking gun. man: i think they had language. man: they were engineering their rocks. "decoding neanderthals," on nova next time. major funding for nova is provided by: supporting nova and promoting public understanding of science. and by the corporation for public broadcasting. and by contributions to your pbs station from: additional funding is provided by millicent bell through: the exploration continues on nova's website, where you can watch this and other nova programs see expert interviews, interactives, video extras and more. follow nova on facebook and twitter, and find us online at pbs.org/nova.

this nova program is available on dvd. to order, visit shoppbs.org, or call 1-800-play-pbs. nova is also available for download on itunes. captioned by media access group at wgbh access.wgbh.org bonneville: i think in season 3, the mood changes and there's a lot of uncertainty still as time creeps forward. lord grantham doesn't deal with the changes very well.

he wants everything to maintain the same and of course, that will never happen. i refuse to be the failure. bonneville: robert isn't a natural businessman. i can tell you that there are some big financial problems on the horizon. we have my mother-in-law coming to stay. robert, aren't you going to kiss me? and any man who has a mother-in-law coming to stay knows that there's fireworks on the horizon. you americans never understand the importance of tradition. yes, we do. we just don't give it power over us. bonneville: and of course, we've got the whole "will they/won't they?" with mary and matthew. i'm looking forward to all sorts of things. don't make me blush. she's still very adamant that things stay the same and that she wants to bring up her children at downton. and matthew is not quite decided upon that. what about us? what about our children? you know i would do anything for this family. anything except help us. bonneville: will edith ever find

proper love and happiness? i do hope to be seeing a bit more of you. bonneville: what will happen to mr. bates? anna: one day, something will occur to us and we'll follow it up and the case against you will crumble. do you never doubt? no. bonneville: and sybil and branson, of course. they come back--an interesting sort of territory to play, because branson having been the chauffeur downstairs is now married to the daughter so that creates a whole social unease and how you cope with that. branson: no, i don't agree and i don't care who knows it! is there any way to shut him up? bonneville: i suppose an overarching theme is the family trying to find its feet again after the upsets of the war. so, there's still plenty to play for. ha ha!