A huge cavern - large enough to house a cathedral - has been dug out beneath the Franco-Swiss border to hold the largest single scientific instrument ever built.
By Dr David Whitehouse
BBC News Online science editor
The Atlas detection equipment will be five stories tall and weigh 7,000 tonnes. Its task is to search for a crucial sub-atomic particle called the Higgs boson, the particle that gives mass to all other particles.
Big enough to house a cathedral
The cavern has just been officially handed over to the scientists by the civil engineering contractors. The scientists will now begin the lengthy and complicated task of installing their detectors.
Atlas, along with three other particle detectors, will form the basis of the large hadron collider (LHC) due to begin operation in April 2005.
The LHC is a circular, underground tunnel 27 kilometres (16.8 miles) long in which two beams of protons travelling in opposite directions will smash into each other; scientists will investigate the debris from the collision.
It is being constructed at Cern, the European Centre for Nuclear Research in Switzerland.
Researchers are especially interested in the rare collisions of the quark particles that make up protons. Because the quarks are so much smaller than the protons, such impacts are exceedingly rare.
The Atlas detector will be huge
Building the cavern for the Atlas detector has been a difficult task. Engineers had to freeze an underground river using liquid nitrogen to create a permafrost through which they could excavate.
The Atlas instrument will surround two colliding proton beams with layers of detectors that will determine the composition of the sub-atomic wreckage from any collision.
Scientists estimate that when the LHC is in operation, it will produce an enormous quantity of data: every second it will yield an equivalent to all the information handled by the world's telecom companies during an entire year.
In the drag
Searching for the Higgs boson will be far, far more difficult than looking for the proverbial needle in a haystack.
If the Higgs particle exists, scientists should detect it in just one in every 10 million, million collisions. With an estimated 800 million collisions every second when the LHC is up to speed, there should be one Higgs particle produced every day.
Where the proton beams will enter
It is estimated that two-thirds of the world's particle physicists will become involved with the LHC.
The Higgs boson was named after the UK physicist Peter Higgs, who postulated its existence more than 30 years ago.
He suggested that the Higgs gives rise to a field through which all other subatomic particles, such as quarks, gluons, photons and electrons, must pass.
As they interact with the field, the particles experience a drag - the more drag, the greater the mass; the less drag, the lighter the particle.
Scientists cannot see the field but they could infer its existence if they can find the particle associated with it in the subatomic debris that is produced when matter is smashed together at high energies in a machine like the LHC.