Human civilisation is lucky to have escaped a volcanic super-eruption powerful enough to blast it back to the Stone Age, say scientists.
New research suggests the average time between the cataclysmic events is much less than previously thought.
Experts now believe the time interval to be only slightly longer than the age of civilisation, dating from the agricultural revolution 12,000 years ago.
Scientists have shown that super-eruptions, which can blanket an entire continent with volcanic ash and alter climate on a global scale, are capable of returning humanity to a pre-civilisation state.
According to geological records the two most recent super-eruptions occurred between 20,000 and 30,000 years ago.
Lead researcher Professor Jonathan Rougier, from the England's University of Bristol, said: "On balance, we have been slightly lucky not to experience any super-eruptions since then.
"But it is important to appreciate that the absence of super-eruptions in the last 20,000 years does not imply that one is overdue. Nature is not that regular.
"What we can say is that volcanoes are more threatening to our civilisation than previously thought."
Like a giant meteor impact, a super-eruption can trigger a "nuclear winter" effect caused by dust thrown into the atmosphere blotting out the sun.
Just one of the volcanic explosions can unleash more than 1,000 gigatons (1,000 billion tons) of erupted mass.
Previous estimates made in 2004 suggested that super-eruptions occurred on average every 45,000 to 714,000 years and posed no immediate threat.
The recalculation, published in the journal Earth and Planetary Science Letters, was based on a statistical analysis of a large database of past volcanic eruptions.
It produces a revised range of 5,200 to 48,000 years with a "best guess" average of 17,000 years.
Yellowstone National Park in the US is the world's leading super-volcano hot-spot.
A dormant volcano in the heart of the park sits on top of a huge reservoir of molten rock and has produced three previous super-eruptions, the last occurring 630,000 years ago.
Experts have warned that Yellowstone could be a ticking time bomb.
David Pyle, Professor in Earth Sciences at Oxford University, said: "In volcanology, many of the easiest questions to ask are the hardest to answer.
"This paper analyses data on past eruptions to ask: 'When will the next globally-significant eruption happen?'
"It turns out that these 'super-eruptions' are still very rare events, but just not quite as rare as previously thought.
"Yellowstone is no nearer erupting today than it was yesterday; and it is still most likely that there won't be another super-eruption anywhere on Earth for thousands of years to come."
Professor David Rothery, from the Open University, stressed that Agung, the volcano now erupting in Bali, had a magma chamber too small to trigger a super-eruption.
"While it may cause local devastation there is no cause for global alarm," he said.