 Judging by the run of successful natural disaster films in the past few years, people are fascinated by the idea of the end of the world. In Danny Boyle's 28 Days Later, a virus ravaged the UK and beyond; an asteroid was the world-ending threat in Deep Impact and armageddon; and climate change got a starring role in The Day after Tomorrow  In the real world, we don't know how the Earth (or humanity) might meet its end or when that will happen. Pondering and predicting the event has usually been a job for the world's great religions: all of them have some idea about how humans will meet their maker. Indeed, "the end" (or judgement day) is usually a deity's way of cleansing our planet, to allow a fresh race of people who are morally purer to repopulate the resulting clean slate. Usually, there is too much sin or debauchery and the time has come to start again.  Stories of brimstone, fire and gods make good tales and do a decent job of stirring up the requisite fear and jeopardy. But made-up doomsday tales pale into nothing, creatively speaking, when contrasted with what is actually possible. Look through the lens of science and "the end" becomes much more interesting.  Since the beginning of life on Earth, around 3.5 billion years ago, the fragile existence has lived in the shadow of annihilation. On this planet, extinction is the norm - of the 4 billion species ever thought to have evolved, 99% have become extinct. In particular, five times in this past 500 million years the steady background rate of extinction has shot up for a period of time. Something - no one knows for sure what - turned the Earth into exactly the wrong planet for life at these points and during each mass extinction, more than 75% of the existing species died off in a period of time that was, geologically speaking, a blink of the eye.  One or more of these mass extinctions occurred because of what we could call the big, Hollywood-style, potential doomsday scenarios. If a big enough asteroid hit the Earth, for example, the impact would cause huge earthquakes and tsunamis that could cross the globe. There would be enough dust thrown into the air to block out the sun for several years. as a result, the world's food resources would be destroyed, leading to famine. It has happened before: the dinosaurs (along with more than half the other species on Earth) were wiped out 65 million years ago by a 10km-wide asteroid that smashed into the area around Mexico.  Monica Grady, an expert in meteorites at the Open University, says it is a question of when, not if, a near-Earth object (NEO) collides with our planet. "Many of the smaller objects break up when they reach the Earth's atmosphere and have no impact. However, a NEO larger than 1km wide will collide with Earth every few hundred thousand years and a NEO larger than 6km, which could cause mass extinction, will collide with Earth every hundred million years. We are overdue for a big one."  Other natural disasters include sudden changes in climate or immense volcanic eruptions. all of these could cause global catastrophes that would wipe out large portions of the planet's life, but, given we have survived for several hundreds of thousands of years while at risk of these, it is unlikely that a natural disaster such as that will cause catastrophe in the next few centuries.  In addition, cosmic threats to our existence have always been with us, even thought it has taken us some time to notice: the collision of our galaxy, the Milky Way, with our nearest neighbour, andromeda, for example, or the arrival of a black hole. Common to all of these threats is that there is very little we can do about them even when we know the danger exists, except trying to work out how to survive the aftermath.  But in reality, the most serious risks for humans might come from our own activities. Our species has the unique ability in the history of life on Earth to be the first capable of remaking our world. But we can also destroy it.  "Existential risks are a relatively novel phenomenon," writes Nick Bostrom, a philosopher and director of the Future of Humanity Institute at Oxford University, in the World Economic Forum's annual publication, Global agenda. "With the exception of a species-destroying comet or asteroid impact (an extremely rare occurrence), there were probably no significant existential risks in human history until the mid-20th century and certainly none that it was within our power to do anything about."  all too real are the human-caused threats born of climate change, excess pollution, depletion of natural resources and the madness of nuclear weapons. We tinker with our genes and atoms at our own peril. Nanotechnology, synthetic biology and genetic modification offer much potential in giving us better food to eat, safer drugs and a cleaner world, but they could also go wrong if misapplied or if we charge on without due care.  Martin Rees, Britain's astronomer royal and former president of the Royal Society, warned in his 2003 book, Our Final Century?, that the odds of human civilisation surviving beyond 2100 are no more than 50%, given the easy access to technologies that could have global impacts, such as biological terrorism, or the potential adverse impacts of molecular nanotechnology.  The first manmade existential risk, said Bostrom, might have been the first detonation of the atomic bomb. "at the time, there was some concern that the explosion might start a runaway chain-reaction by 'igniting' the atmosphere. although we now know that such an outcome is physically impossible, an existential risk was nevertheless present then."  Potential points of danger continue to come from the more successful achievements of our recent past. Our society is connected and computerised like never before and this has brought us big benefits in terms of trade, access to knowledge and education and better communications. But those same interconnections can spread viruses (human and computer) ever faster. a skilled terrorist cell (or intelligent machine) could compromise power systems, steal or delete financial data and wreck supply chains, all of which are crucial for the modern world to function. a failure in a digital system in the United States can spread to China or australia in seconds.  It is perhaps ironic that the shadow of potential threats becomes ever longer the more light we shed on our understanding of the universe.  Imagine that we took some of the most learned figures of the enlightenment period in western Europe - Isaac Newton, say, or Francis Bacon, or Bishop George Berkeley - and asked them how they thought the world would come to an end. There might be tales of divine intervention (Newton believed doomsday would be in the 21st century, calculated from clues in the Bible), or the idea that a bloody war would end up causing so many casualties that nations would suffer and wither away. There might be serious consideration of other fantastical theories, but none of these clever people could have told you about the doomsday potential of nuclear bombs, or black holes, or rising sea levels due to climate change.  You can only know that the world could pop out of existence in a bout of vacuum decay, and be wiped out in a blink, if you know about quantum particles and the evolution of the universe since the big bang. We are beginning to understand that what we conceive of as "time" might one day disappear from our universe, giving us no sense of movement or direction.  and let us hope we never run into a clump of the deadly strangelet matter anywhere in the universe. This is a substance nominally so very close to being made of the same stuff that makes up everything we see around us, yet coldly destructive of our way of life.  Jason Matheny, a program manager at the US government's Intelligence advanced Research Projects activity, routinely considers potential ways that humanity might be threatened. In a 2007 article for the journal Risk analysis, he pondered the inevitable death of the sun. "In one billion years, the sun will begin its red giant stage, increasing terrestrial temperatures above 1,000 degrees, boiling off our atmosphere, eventually forming a planetary nebula, making Earth inhospitable to life," he wrote. "If we colonise other solar systems, we could survive longer than our sun, perhaps another 100 trillion years, when all stars begin burning out. We might survive even longer if we exploit non-stellar energy sources."  Which all sounds very positive. But the universe has some further tricks up its sleeve. It is hard to imagine, wrote Matheny, how humanity will survive beyond the decay of nuclear matter, which is expected in 10[cubed][squared] to 1041 years. "Physics seems to support Kafka's remark that there is infinite hope, but not for us. While it may be physically possible for humanity or its descendents to flourish for 1041 years, it seems unlikely that humanity will live so long. Homo sapiens has existed for 200,000 years. Our closest relative, Homo erectus, existed for around 1.8 million years. The median duration of mammalian species is around 2.2 million years."  Should any of this doomsaying concern us, particularly in a credit-crunched world? Yes, argues Bostrom. "attempts to quantify existential risk inevitably involve a large helping of subjective judgment. and there may be a publication bias  