One of the scariest things imaginable -- and the threat is real. A supervolcano is a volcano like any other -- but several orders of magnitude bigger than any of the more familiar volcanoes. Whereas even big volcanoes like Mount St Helens or Krakatoa have craters up to a mile of two across, it isn't widely known that almost the whole of Yellowstone National Park in the USA sits within the crater of one of the world's most powerful supervolcanoes.

Only discovered in recent years, supervolcanoes are formed when the magma which creates a normal volcano becomes completely trapped under the ground. Unable to find any way at all to vent, the molten rock becomes placed under increasing pressure, until eventually, instead of finding its way to the surface through a thin lava tube like a normal volcano, it explodes with a force equivalent to that of an asteroid strike on the earth's surface. But whereas asteroids could possibly be diverted or blown up, there is nothing at all can be done about supervolcanoes. Unlike normal volcanoes, which form mountains to give away their presence, supervolcanoes erupt with such force and devastation that they actually create a small caldera or collapsed crater, which is hard to detect. Yellowstone sits within one such caldera:

A report by the BBC television programme Horizon on the Yellowstone supervolcano says it is overdue for an eruption. Yellowstone has gone off roughly once every 600,000 years. Its last eruption was 640,000 years ago. Professor Bill McGuire, of the Benfield Greig Hazard Research Centre at University College, London, said "We're getting ready for another eruption, unless the system has blown itself out. But the ground surface deformation and other signs measured by satellite suggest it's still active, and on the move". Ground temperature readings taken at Yellowstone over the past 50 years have shown an increase of nearly 2 degrees Celsius and satellite height recordings indicate that the ground is rising, by over a foot a year in some parts of Yellowstone, indicating that the pressure of the magma below ground is building up to breaking point.

If the Yellowstone supervolcano goes off it will not only wipe out life across half the North American continent, but it will throw tens of cubic miles of rock, ash and dust into the stratosphere, triggering a nuclear winter that could last for up to 5 years. This will of course completely devastate the earth's eco-system and could plunge civilisation into famine or war.

See, I told you this was one of the scariest things imaginable. And there's absolutely nothing anyone can do about it. Be afraid, be VERY afraid...

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