Part of the Compact Muon Solenoid particle detector is lowered into place several hundred feet below ground. COURTESY CERN

The European Organization for Nuclear Research, known as CERN, has its offices on the outskirts of Geneva, in an area once devoted to dairy farms and now given over to sprawl. The offices occupy several dozen buildings, some of them in Switzerland and the remainder, a few hundred yards away, in France. The buildings are reachable by roads with names like Route Bohr, Route Schrödinger, and Route Curie. By the entrance to the complex, there is a museum—nearly empty the day I visited—that attempts to make particle physics comprehensible to the general public. Behind that there is a park where bits of old cyclotrons are displayed, like playground equipment from Mars.

If you think of the sciences as a tower, with one field resting on another until you reach, say, botany or physiology, then particle physics represents the bottommost floor. The first key experiment was conducted in 1909, under the direction of Ernest Rutherford. When Rutherford shot alpha particles at a wafer-thin sheet of gold foil, a small proportion of the particles bounced right back, a phenomenon that he described as “almost as incredible as if you fired a fifteen-inch shell at a piece of tissue paper and it came back to hit you.” Rutherford’s work led to the realization that most of an atom’s mass was concentrated in a tiny area, the nucleus. “All science is either physics or stamp-collecting,” he is supposed to have said.

Since Rutherford’s discovery, particle physics has provided one extraordinary—if increasingly implausible-sounding—revelation after another: first protons and neutrons, then antimatter, gluons, neutrinos, and quarks. In 1967, the existence of particles to mediate the weak force, which is responsible for radioactive decay, was theorized; in 1983, at CERN, these particles—the W and the Z—were observed and their properties measured. In 1977, the existence of what became known as the “top” quark was predicted; in 1995, at Fermilab, in Illinois, it, too, was found.

And yet, for all its triumphs, the field has been haunted by failure. The more physicists have learned about the way matter behaves at its most fundamental level, the more acutely they have become aware that something—a big something—is missing from their accounts. Among the many possibilities proposed for what’s often called “new physics” is that the universe actually consists of tiny strands (or strings) of energy; that it contains several dimensions beyond those that we perceive; that it is full of mysterious particles—“sparticles”—that have yet to be detected; that it is not a universe at all but a multiverse; and that it began not with a bang but with a splat.

Sometime in the next few months, physicists at CERN will finish preparations for the most ambitious particle-physics experiment ever, which will be conducted in an apparatus modestly referred to as the Large Hadron Collider, or L.H.C. The L.H.C. fills a circular tunnel seventeen miles in circumference. To get from one side of it to the other, it is necessary to drive through several towns, and then descend three hundred feet in an elevator. Alternatively, it is possible to ride through the tunnel in one of the dozens of bicycles CERN provides for its staff, but in that case a supply of emergency oxygen is required.

The L.H.C. is considered the best—some would say the only—hope for testing the theories of “new physics” against material reality. Once the collider begins operating at full power—in early 2008, if all goes well—nearly half the particle physicists in the world will be involved in analyzing its four-million-megabyte-per-hour stream of data. Few events in the history of science have had a bigger buildup. It’s been suggested that the L.H.C. will unlock the secrets of the universe or, barring that, prove this ambition to be hopeless.

The L.H.C. is a kind of Babel built underground. Dozens of countries have manufactured its components, and dozens more have lent manpower and expertise. (Some contracts went to Russian physicists who previously worked for the Soviet military; in this way, the collider has provided a livelihood for scientists whose employment options might otherwise include selling nuclear secrets.) When I ate in CERN’s lunchroom, I heard people speaking English, French, German, and Italian, as well as several languages that I couldn’t identify. The place was so crowded that it took me five minutes to pay for a cup of coffee, proving the elemental truth that man can build a superconducting collider but not a functional cafeteria.

CERN’s chief scientific officer, Jos Engelen, is from the Netherlands. He serves under the director general, who is from France, and alongside the chief financial officer, who is from Germany. I went to speak to Engelen in his office; behind his desk a chart indicated when the various parts of the collider are supposed to be completed. It was a crazy quilt of multicolored blocks, with lines radiating in all directions. Engelen greeted me with a half-ironic cheerfulness that struck me as very Dutch. Among his responsibilities is dealing with the frequent calls and letters CERN receives about the possibility that the Large Hadron Collider will destroy the world. When I asked about this, Engelen picked up a Bic pen and placed it in front of me.

“In quantum mechanics, there is a probability that this pen will fall through the table,” he said. “All of a sudden, it will be on the floor. Because it can behave as a wave, it can go through; we call that the ‘tunnel effect.’ If you calculate the probability that this happens, it is not identical to zero. It is a very small probability. But it never happens. I’ve never seen it happen. You have never seen it happen. But to the general public you make a casual remark, ‘It is not identical to zero, it is very small,’ and . . . ” He shrugged.

Worries about the end of the planet have shadowed nearly every high-energy experiment. Such concerns were given a boost by Scientific American—presumably inadvertently—in 1999. That summer, the magazine ran a letter to the editor about Brookhaven’s Relativistic Heavy Ion Collider, then nearing completion. The letter suggested that the Brookhaven collider might produce a “mini black hole” that would be drawn toward the center of the earth, thus “devouring the entire planet within minutes.” Frank Wilczek, a physicist who would later win a Nobel Prize, wrote a response for the magazine. Wilczek dismissed the idea of mini black holes devouring the earth, but went on to raise a new possibility: the collider could produce strangelets, a form of matter that some think might exist at the center of neutron stars. In that case, he observed, “one might be concerned about an ‘ice-9’-type transition,” wherein all surrounding matter could be converted into strangelets and the world as we know it would vanish. Wilczek labelled his own suggestion “not plausible,” but the damage had been done. “BIG BANG MACHINE COULD DESTROY EARTH” ran the headline in the London Times. Brookhaven was forced to appoint a committee to look into this and other disaster scenarios. (The committee concluded that “we are safe from a strangelet initiated catastrophe.”)