Last December, Clive Oppenheimer stood close to the summit of Mount Nabro, a 7,000ft (2,133m) active volcano on the border of Eritrea and Ethiopia, and looked down at the ashen soil. Scattered around his feet were axeheads, hammers and other tools made from shiny black volcanic rock by hunter-gatherers more than 10,000 years ago.

¡°Rubbish from the Palaeolithic era was littered around the place,¡± recalls Oppenheimer, at the University of Cambridge. ¡°Each of these obsidian tools has its own chemical fingerprint and sometimes they can be found hundreds of miles away from where they were made. There are also lots of interesting engravings from the time showing cattle, which is a testament to when the area was a major migration route¡± and brought people from all over Africa to a region widely regarded as the birthplace of mankind.

Oppenheimer had been led to the peak in the East African Rift Valley by his research on gaseous emissions; the volcano had erupted in 2011 after thousands of years of dormancy. But it is not only science that draws people to Mount Nabro. The detritus on its slopes is testament, Oppenheimer believes, to the eternal power of volcanoes to attract human beings in general.

He first set out his thoughts on this topic in his 2011 book Eruptions that Shook the World, but they are now set to receive a much wider airing. That is because his book piqued the interest of maverick film-maker Werner Herzog, whose astonishing films, such as Fitzcarraldo and Aguirre: the Wrath of God, and documentaries, such as Grizzly Man and Lessons of Darkness, have similarly meditated on the fragility of man in the face of nature. Having recently played the villain in the first Jack Reacher movie and directed Queen of the Desert, a lavish biopic about the Arabist Gertrude Bell, the German auteur decided to return to documentary-making.

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Oppenheimer had first met Herzog in 2006, when the director was filming Encounters at the End of the World: an Oscar-nominated documentary about life in Antarctica, where Oppenheimer spends several months each year monitoring the 3,450m Erebus volcano. ¡°He was filming at our base camp. We got on very well, and he told me that he¡¯d wanted to make a documentary about volcanoes for some time,¡± says Oppenheimer, who also fondly recalls Herzog¡¯s 1977 short film La Soufri¨¨re, about the refusal by inhabitants of Guadeloupe to evacuate the Caribbean island after a volcano was predicted to erupt there. ¡°We talked in vague terms about our aspirations to film volcanoes, kept in touch over the years and I sent him a copy of my book when it was published.¡±

Having pushed the project for several years, Herzog finally secured funding for Into the Inferno from Netflix, on whose streaming service it will be shown from 28 October, before a limited theatrical release. Filmed between August 2015 and May 2016, the film features Oppenheimer leading Herzog and his film crew on expeditions to some of the world¡¯s most remote and dangerous volcanoes, from Iceland to the deepest Sahara.

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¡°Taking a film crew to volcanoes presented new challenges to me as they do not see the same risks as a volcanologist,¡± observes Oppenheimer. ¡°There were a couple of more hazardous volcanoes where I was very aware of the risks we were taking.¡±

The very active volcano of Mount Yasur on Vanuatu¡¯s Tanna Island in the South Pacific was the scene of ¡°one of the riskier shoots¡±, since ¡°it goes bang every minute or so, showering the ground with lava bombs, so it was a question of staying out of range¡±. The 52-year-old volcanologist had to balance the need for spectacular footage with his team¡¯s safety, as well as possible opprobrium from academic peers who disdain such sensationalisation of their subject.

¡°Some film-makers have passed themselves off as volcanologists and gone right down into volcanoes while they were active,¡± he says. ¡°I am tainted by that sort of film-making now that I¡¯ve crossed over into this area ¨C I do feel a bit uncomfortable about it.¡± On the other hand, he notes that ¡°the advent of the impact agenda and social media means this generation [of academics] is a lot more comfortable with this type of crossover into popular science¡±.

Moreover, Into the Inferno is ¡°more anthropological and scientific¡± than previous volcano films, exploring the pivotal role that volcanoes have played over the millennia in shaping everything from farming and migration to trade and religion. ¡°It is a lot about faith,¡± Oppenheimer says. ¡°I didn¡¯t want to make [a film] about the doom and gloom [volcanoes] can cause, with a bit of pyrotechnics thrown in ¨C although we [do] have some of that.¡±

One scene shows the film crew arriving at an archaeological dig in the East African Rift Valley. The skeleton of a man, believed to be 15,000 to 20,000 years old, is being exhumed by a team led by palaeoanthropologist Tim White of the University of California, Berkeley, best known for his work on the 3.2 million-year-old skeleton of the hominin ¡°Lucy¡±, found in the nearby Awash Valley in 1974.

¡°It¡¯s not a coincidence¡±, in Oppenheimer¡¯s view, ¡°that all the main fossil finds of early man have occurred in the East African Rift.¡± This is because ¡°thanks to volcanoes, you have a natural resource base ¨C water in big lakes, animals that come to drink there and a topography between the shore and the plateau that allows you to corral prey. You also have volcanic glass that can be used to make tools.¡±

He also thinks that early humans would naturally have developed ¡°stories, mythologies and cosmologies connected to volcanoes because they are so awesome¡±. This point is illustrated in the film by an interview with a modern-day African shaman.

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Professor Clive Oppenheimer

For 'Into the Inferno', Oppenheimer and Herzog visited Yasur (above), which ¡®goes bang every minute or so, shower?ing the ground with lava bombs¡¯, and North Korea¡¯s Mount Paektu, ¡®a very tranquil and beautiful place¡¯

¡°He explained about the ¡®red water¡¯ he had seen moving in the volcano, which he knew clearly wasn¡¯t water, but if you don¡¯t have the science to explain what is happening, then you start to create stories about it,¡± Oppenheimer says.

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Similar mythologies were found in other volcano-dominated communities that he visited with Herzog, he adds: ¡°It helps us to examine the idea of faith and how new religions may emerge.¡±

Oppenheimer and Herzog also travelled to the Tibesti mountains on the border of Libya and Chad to visit the extinct super-volcanoes of Trou au Natron, Pic Toussid¨¦ and Tieroko, which caused global devastation when they last erupted, between 100,000 and 200,000 years ago. But while there was no threat from lava, the trip was still risky given the volatile political situation in the region, to which the Foreign and Commonwealth Office advises UK nationals not to travel.

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¡°The area is a bulwark against [Islamist group] Boko Haram in West Africa and Islamic State in Libya, but there are still security issues,¡± admits Oppenheimer. These include uncleared minefields, ¡°but we worked very closely with [local] Tubu guides and drivers and felt highly confident in their knowledge of the terrain and of unsafe areas to avoid,¡± Oppenheimer says. Nevertheless, ¡°it is still something that is on your mind when exploring areas away from the main trails¡±.

Although Herzog is known for his physically gruelling shoots (an enormous ship was winched up a hill in the Peruvian jungle in Fitzcarraldo, filmed in 1982), Oppenheimer also had concerns for the health of the ¡°amazingly fit¡± 74-year-old director in the blazing African heat.

¡°We were climbing volcanoes in the middle of the Saharan desert, so we needed to be vigilant,¡± he says. And even when there is no danger of eruption, ¡°volcanoes are inherently risky places¡­because if you trip and sprain your ankle in a desert or in Antarctica you can get into trouble very quickly ¨C that window to get to a hospital or find treatment in time is much smaller as you are so far away from safety¡±.

Oppenheimer has also faced ¡°troubling situations in Ethiopia on a couple of occasions, when we encountered armed men who did not have our best interests at heart. Resolution required diplomacy and negotiation in each case, but there were edgy moments when the men detaining us started posturing with their guns.¡±

Flying lava bombs in the Antarctic have also posed dangers. ¡°In that situation, you need to take precautions to mitigate the risk, which means setting up the equipment and getting out of there,¡± Oppenheimer says. The combination of gravity and ice is also perilous in that part of the world, he adds. ¡°There are some slopes of slick ice that we traverse from time to time, where a fall could lead to a more or less serious injury.¡± All of that said, his memories of hairy moments on potholed roads around the world suggest that simply driving is often the greatest hazard.

The practical and bureaucratic challenges facing Herzog and Oppenheimer in North Korea were mundane by comparison. Oppenheimer describes the sacred Mount Paektu as ¡°a very tranquil and beautiful place ¨C you can see right across the Chinese border. On the Chinese side, there are thousands of tourists and on the North Korean side there are thousands of pilgrims as it was the birthplace of the [communist] revolution.¡±

It is very rare for Western film crews to be permitted to shoot there. Herzog¡¯s application was presumably granted on account of Oppenheimer¡¯s regular trips to Pyongyang over the past five years to advise on the current state of the volcano, which is said to pose a ¡°very real threat¡± after remaining dormant for centuries. Its last eruption in AD946 was so violent that it formed a 3.1-mile (5km) crater and showered ash on Japan ¨C almost 683 miles (1,100km) away.

Communication in North Korea is ¡°challenging¡±, Oppenheimer concedes. His team ¡°have to use translators, and the North Koreans haven¡¯t been exposed to the latest international science journals, so they are a long way from the cutting edge in terms of volcanology. It¡¯s not a completely closed country, but the lack of Western banks [also] means we have to carry wads of cash when we go in.¡±

London-raised Oppenheimer¡¯s interest in volcanoes has been burning fiercely since it was first kindled during a gap-year trip prior to studying geology at Cambridge. ¡°I travelled round Indonesia with a book by the [Open University] volcanologist Peter Francis,¡± he recalls. ¡°I was later going to do a PhD in seismology, but happened to see an opportunity in volcanology and Peter became my supervisor.¡± But with two young daughters at home in Cambridge, does Oppenheimer expect to hang up his mountaineering boots at some point and keep to the lab? Not for a while, he replies ¨C although spending time in the lab is ¡°absolutely crucial¡± for his research.

¡°I have a reputation in my department for not being around,¡± he acknowledges, ¡°but I am semi-nomadic [by nature] and I get restless if I am in the same place for too long¡­Fieldwork is also what I¡¯m good at ¨C it¡¯s a skill, as you never know what the access will be like or what a volcano will be doing, [and] it can be hard when acidic gases are corroding your equipment.¡±

Such difficult conditions mean that Oppenheimer is not always sure what he has found until he analyses the data months later ¨C but that, for him, is part of the joy of fieldwork.

¡°I¡¯m really against the idea that you must have a hypothesis and you find the data to prove or disprove it,¡± he says. ¡°I like the serendipity of working in the field.¡±

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Werner Herzog¡¯s Into the Inferno will premiere on Netflix on 28 October before a limited release in cinemas worldwide. Its UK premiere will be at the Cambridge Film Festival on 27 October.