Scientists today "switched on" the most powerful particle accelerator ever built in an attempt to answer some of the biggest unanswered questions in physics.
Despite some "small electrical problems" overnight, the £5 billion Large Hadron Collider (LHC) was switched on at 8.39am BST after a slight delay.
Scientists later announced they had succeeded in sending a beam of protons all the way round the collider in one direction. The next step will be to test the other direction.
As experiments build, the LHC will smash protons into each other at energies up to seven times greater than any achieved before.
No one knows precisely what will come tumbling out of the primordial soup of disintegrating protons, but the scientists have dismissed suggestions that the experiment could somehow cause the end of the world.
The LHC could help scientists explain mass, gravity, mysterious "dark matter" and why the universe looks the way it does.
It could also produce the first evidence of extra spatial dimensions and even create mini-black holes that blink in and out of existence in a fraction of a second.
The LHC, a colossal machine housed in a 27 kilometre (17 mile) tunnel under 100 metres of rock, straddles the borders of Switzerland and France between Lake Geneva and the Jura mountains.
Beams of protons will be accelerated in opposite directions through the ring-shaped tunnel, which is supercooled to just 1.9 degrees above absolute zero (minus 271C), the lowest temperature allowed by nature.
Reaching velocities of 99.99 per cent of the speed of light, each beam will pack as much energy as a Eurostar train travelling at 150 kilometres per hour.
The particles will be brought together in four huge "detectors" placed along the ring. Each detector is like a giant microscope, designed to probe deeper into the heart of matter than has ever been possible before.
Concerns have been voiced - in particular by German chemist Professor Otto Rossler - that black holes created by the LHC will grow uncontrollably and "eat the planet from the inside".
But those involved in the project insist they have reviewed all the evidence and concluded that it poses no risk to the universe.
Particle physicist Dr James Gillies, a spokesman for the LHC, said: "We have received a lot of worried calls from people about it.
"There's nothing to worry about, the LHC is absolutely safe because we have observed nature doing the same things the LHC will do.
"Protons regularly collide in the earth's upper atmosphere without creating black holes.
"What we are looking at is a global community representing 10,000 people working in 500 universities in 80 countries, none of whom has the slightest worry about risks of this kind.
"Then we have a retired German chemist who has never published a paper in this field in his life, who has come up with this theory.
"We are very excited about the project. We hope to learn more about this wonderful universe of ours."
The eyes of the world were on LHC project leader Dr Lyndon Evans, from Aberdare in south Wales, in the tense minutes before the machine was "switched on".
Looking relaxed in a short-sleeved shirt and jeans, Dr Evans counted down the last few seconds before the first beam of protons was put into the LHC.
"Five, four, three, two, one, zero - nothing," he joked before a blip appeared on a computer monitor signalling that the long years of hard work had paid off and the machine was working.
Dr Evans, whose father was a coalminer, said: "This is really the biggest and most complex scientific project ever undertaken, and you cannot do a thing like this without engineers and applied scientists of very top quality."
Skills Secretary John Denham hailed the launch of the LHC today as an "extraordinary moment".
Noting that the project had taken two decades to come to fruition, he joked: "My lab technique used to be bad but I used to get set up quicker than that."
Mr Denham said theoretical research like this often produced practical benefits but said this was not the only concern of the Government in providing funding.
He said: "We do this fundamentally because we need to know. We need to know as human beings because we have a curiosity, an intellectual excitement."
Turning on the LHC was nothing like as simple as flipping a switch.
A chain of smaller accelerators, built for earlier projects, were first used to speed up the proton beams to the point where they could be injected into the machine.
The start of the process involved a bottle of hydrogen gas no bigger than a fire extinguisher.
Hydrogen atoms were stripped of their electrons to produce streams of protons that are fed into accelerators of increasing size.
The last link in the chain before the LHC, the Super Proton Synchrotron (SPS), is buried underground and covers a distance of seven kilometres.
Timing between the SPS and the LHC has to be accurate to within a fraction of a nanosecond.
Today's "switch on" involved transferring a beam from the SPS to the LHC so that it is circulating around the machine in a stable fashion.
The first particle collisions are likely to take place within a few weeks.
In some cases teams of more than 2,000 collaborating scientists will be sifting and analysing data from the machine.
Most will not be at the LHC's operating base at CERN, the European nuclear research organisation, in Geneva.
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A revolutionary computer network called the "Grid" - the next step beyond the World Wide Web - will make it possible for scientists all over the world to share huge amounts of processing power and carry out much of the work on their PCs.
The cost of the LHC is mainly shared by CERN's 20 European member states, which include Britain. Six "observer" nations, including the US, Russia and Japan, make significant contributions.
CERN estimates the total cost of the project to be 10 billion Swiss francs, or £5 billion. The material cost alone is put at £2.6 billion.
Britain's direct contribution to the LHC each year is £34 million.
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The Large Hadron Collider: End of the world, or God's own particle?
A bewildered Cole Moreton goes in search of the science behind the spin...
Yes, but what is it? That has been many people's reaction to the furore over the Large Hadron Collider. The biggest, most expensive experiment in history is attracting both scientific hyperbole and hysteria. Some say it will reveal the universe's secrets and lead to the elusive Theory of Everything. A few fear that unleashing unimaginable power beneath the Swiss countryside will result in the end of the world. But how? And what do all these words mean?
Is an understatement. A giant circular tunnel, with several loops, stretches for 27km under the land between France and Switzerland. One of its experimental chambers is bigger than the nave of Westminster Abbey.
The name for one of the types of particle that make up an atom. These tiny bits of energy will be propelled by giant magnets around the tunnel circuit at almost the speed of light.
Is what they will do when they meet other hadrons being beamed in the opposite direction, at the same great speed. The resulting explosion will create 100,000 times more heat than the sun, apparently. Thankfully, it will only happen for a moment, in an area a billion times smaller than a speck of dust.
Pronounced "sern". The French acronym for the European Organisation for Nuclear Research, which built the £5bn collider. The money came from 20 countries, including Britain, which has played a leading role.
The Big Bang
Is what they are trying to recreate. Or rather what happened a trillionth of a second after the universe was created by an explosion, 13.7 billion years ago. For that tiny moment, it is believed everything was molten plasma. This cooled to create everything we see around us. The hope is that by remaking the moment, in miniature, the scientists will be able to see things that are invisible now.
The God Particle
Big name, very small thing; and the first great discovery they hope to make. It is believed we have only detected a quarter of the particles in everything. We don't, for example, know why things have mass. (To get a feeling for what that is, hit yourself over the head with an inflatable hammer, then a real one. The one that hurts has more mass.) In 1964 Professor Peter Higgs of Edinburgh University predicted an unseen particle that provided mass (its official name is a Higgs boson). The hope is it will be detected for the first time. Other possible revelations include so-called dark matter, which in theory "stretches through space like an invisible skeleton".
The Theory of Everything
The Holy Grail of science. A unifying theory providing one explanation for the forces at work in the natural world, from the nucleus of an atom to the movements of the planets. Sounds like alchemy to non-scientists, but some very respectable minds believe it is possible, and that the collider may show the way.
The End of the World
Some scientists, on the other hand, went to the European Court for Human Rights to try to stop the collider being turned on. They fear it may create a black hole – which would certainly violate our rights by sucking the planet into... well we don't really know. Professor Sir Chris Llewellyn Smith of Cern says: "The chance we produce a black hole is minuscule." Which is not all that reassuring. But he adds: "Even if we do, it can't swallow up the Earth." It would be too small, and disappear in moments. In any case, they will only send the hadrons in one direction this week. The collisions start in October. Until then, at least, we're not all doomed.