Particle physicists were to fire up the world's biggest atom-smasher on Wednesday in a mission to answer some of the most perplexing questions in the cosmos.
Built in a tunnel 100 metres (325 feet) below ground in a complex straddling the French-Swiss border, the Large Hadron Collider (LHC) is designed to accelerate sub-atomic particles to nearly the speed of light and then smash them together.
The collision will briefly stoke temperatures 100,000 times hotter than the Sun, fleetingly replicating conditions which prevailed in split-seconds after the "Big Bang" that created the Universe 13.7 billion years ago.
In this primordial soup, novel particles may lurk that will resolve mysteries clouding our understanding of fundamental matter, scientists say.
"This machine will probably bring unexpected results that could turn particle physics on its head," French astrophysicist Hubert Reeves said on Tuesday.
It has required nearly two decades, six billion Swiss francs (3.76 billion euros, 5.46 billion dollars) and 5,000 scientists, engineers and technicians from nearly three dozen countries to bring the LHC to fruition.
At 9:30 am (0730 GMT) on Wednesday, the first protons will be injected into the 27-kilometre (16.9-mile) ring-shaped tunnel at the headquarters of the European Organisation for Nuclear Research (CERN).
The operation will kick off a long and cautious commissioning process, testing equipment and procedures, before starting experiments a matter of weeks from now.
When all is ready, the LHC will whizz two parallel beams of protons, one clockwise and the other anticlockwise, around the tunnel.
Superconducting magnets cooled close to absolute zero, the chill of deep space will then steer the beams so that they converge inside four chambers, like racing cars in a chicane.
Some protons are bound to collide, and subatomic wreckage from the smashup will fly into the detectors, leaving a calling-card trace of their identity.
Over the years in which will the LHC will operate, masses of data will spew from these collisions and will be closely scrutinised by universities and laboratories around the world.
The Holy Grail will be finding a particle, called the Higgs Boson after British physicist Peter Higgs, who devised the theory of its existence in 1964.
The "Higgs" would explain how particles acquire mass, and some particles are more massive than others.
The idea is that these particles exist in a sort of invisible background field. Other particles passing through the Higgs field would acquire mass, like feathers passing through treacle.
Another big challenge will be testing the theory of supersymmetry, which postulates that the members of the known bestiary of sub-atomic particles have related, but more massive, counterparts.
Such particles could explain the unsettling discovery of recent years that visible matter only accounts for some four percent of the Universe. Enigmatic phenomena called dark matter and dark energy account for the rest.
Before the startup, Internet-driven rumours said the LHC would create black holes or a nasty hypothetical particle called a strangelet that would gobble up the planet.
CERN has commissioned a panel to verify its calculations that such risks are, by any reasonable thinking, impossible. France too has carried out its own safety probe.