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 the scientists 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,” project leader Lyn Evans said.

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 are theorized to make up more than 96 percent of the universe, and hope to glimpse a 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.

The safety of the collider, which will generate energies seven times higher than its most powerful rival, at Fermilab near Chicago, 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.

By contrast, 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, including one Nobel laureate, endorsed it.

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.”

One of the plaintiffs, Walter L. Wagner, a physicist and lawyer, said Wednesday that CERN’s safety report, released June 20, “has several major flaws,” and his views on the risks of using the collider had not changed.

On Tuesday, U.S. Justice Department lawyers representing the Department of Energy and the National Science Foundation filed a motion to dismiss the case.

The two agencies have contributed $531 million to building the collider, and the NSF has agreed to pay $87 million of its annual operating costs. Hundreds of American scientists will participate in the research.

The lawyers called the plaintiffs’ allegations “extraordinarily speculative,” and said “there is no basis for any conceivable threat” from black holes or other objects the LHC might produce. A hearing on the motion is expected in late July or August.

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

And so far, Earth has survived.

“The LHC is only going to reproduce what nature does every second, what it has been doing for billions of years,” said John Ellis, a British theoretical physicist at CERN.

Critics like Wagner 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 Earth.

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

The leafy campus of CERN hardly seems like ground zero for doomsday. And locals don’t seem overly concerned. Thousands attended an open house here this spring.

“There is a huge army of scientists who know what they are talking about and are sleeping quite soundly as far as concerns the LHC,” said project leader Evans.