Associated Press

MEYRIN, Switzerland - The most powerful atom-smasher ever built could make some bizarre discoveries, such as invisible matter or extra dimensions in space, after it is switched on in August.

But some critics fear the Large Hadron Collider could exceed physicists' wildest conjectures: Will it spawn a black hole that could swallow Earth? Or spit out particles that could turn the planet into a hot dead clump?

Ridiculous, say scientists at the European Organization for Nuclear Research, known by its French initials CERN - some of whom have been working for a generation on the $5.8 billion collider, or LHC.

"Obviously, the world will not end when the LHC switches on," said project leader Lyn Evans.

David Francis, a physicist on the collider's huge ATLAS particle detector, smiled when asked whether he worried about black holes and hypothetical killer particles known as strangelets.

"If I thought that this was going to happen, I would be well away from here," he said.

The collider basically consists of a ring of supercooled magnets 17 miles in circumference attached to huge barrel-shaped detectors. The ring, which straddles the French and Swiss border, is buried 330 feet underground.

The machine, which has been called the largest scientific experiment in history, isn't expected to begin test runs until August, and ramping up to full power could take months. But once it is working, it is expected to produce some startling findings.

Scientists plan to hunt for signs of the invisible "dark matter" and "dark energy" that make up more than

96 percent of the universe, and hope to glimpse the elusive Higgs boson, a so-far undiscovered particle thought to give matter its mass.

The collider could find evidence of extra dimensions, a boon for superstring theory, which holds that quarks, the particles that make up atoms, are infinitesimal vibrating strings.

The theory could resolve many of physics' unanswered questions, but requires about 10 dimensions - far more than the three spatial dimensions our senses experience.

A CERN team this month issued a report concluding that there is "no conceivable danger" of a cataclysmic event. The report essentially confirmed the findings of a 2003 CERN safety report, and a panel of five prominent scientists not affiliated with CERN endorsed its conclusions.