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 safety of the collider, which will generate energies seven times higher than its most powerful rival, has been debated for years. The physicist Martin Rees has estimated the chance of an accelerator producing a global catastrophe at one in 50 million – long odds, to be sure, but about the same as winning some lotteries.

Critics of the LHC filed a lawsuit in a Hawaiian court in March seeking to block its startup, alleging that there was “a significant risk that … operation of the Collider may have unintended consequences which could ultimately result in the destruction of our planet.”

In rebutting doomsday scenarios, CERN scientists point out that cosmic rays have been bombarding the earth, and triggering collisions similar to those planned for the collider, since the solar system formed 4.5 billion years ago.

Critics have said the collisions caused by accelerators could be more hazardous than those of cosmic rays.

Both may produce micro black holes, subatomic versions of cosmic black holes – collapsed stars whose gravity fields are so powerful that they can suck in planets and other stars.

But micro black holes produced by cosmic ray collisions would likely be traveling so fast they would pass harmlessly through the earth.

Micro black holes produced by a collider, the skeptics theorize, would move more slowly and might be trapped inside the earth’s gravitational field – and eventually threaten the planet.

John Ellis, a British theoretical physicist at CERN, said doomsayers assume that the collider will create micro black holes in the first place, which he called unlikely.