If you were to dig a hole 300 feet straight down from the center of the charming French village of Crozet, you'd pop into a setting that calls to mind the subterranean lair of one of those James Bond villains. A garishly lit tunnel ten feet in diameter curves away into the distance, interrupted every few miles by lofty chambers crammed with heavy steel structures, cables, pipes, wires, magnets, tubes, shafts, catwalks, and enigmatic gizmos.

This technological netherworld is one very big scientific instrument, specifically, a particle accelerator-an atomic peashooter more powerful than any ever built. It's called the Large Hadron Collider, and its purpose is simple but ambitious: to crack the code of the physical world; to figure out what the universe is made of; in other words, to get to the very bottom of things.

Starting sometime in the coming months, two beams of particles will race in opposite directions around the tunnel, which forms an underground ring 17 miles in circumference. The particles will be guided by more than a thousand cylindrical, supercooled magnets, linked like sausages. At four locations the beams will converge, sending the particles crashing into each other at nearly the speed of light. If all goes right, matter will be transformed by the violent collisions into wads of energy, which will in turn condense back into various intriguing types of particles, some of them never seen before. That's the essence of experimental particle physics: You smash stuff together and see what other stuff comes out.

By the early 1930s Ernest Lawrence had invented the first circular particle accelerator, or "cyclotron." It fit in his hand.

Now the U.S. government has an accelerator that's hidden beneath several square miles of tallgrass prairie and a small herd of buffalo at its Fermilab facility west of Chicago. When you drive on the Junipero Serra freeway near Palo Alto, California, you pass directly over a two-mile linear accelerator. The LHC crosses the border between two countries. There are still physicists who do tabletop physics—who try to get big answers with modest means—but realistically you need huge, powerful, energetic devices to pry open the fabric of reality.

We know things today that Einstein, Rutherford, Max Planck, Niels Bohr, Werner Heisenberg, and the rest of the great physicists of a century ago couldn't have imagined. But we're nowhere near a final theory of physical reality. Molecules are made of atoms; atoms are made of particles called protons, neutrons, and electrons; protons and neutrons (which are the "hadrons" that give the collider its name) are made of odd things called quarks and gluons—but already we're into a fuzzy zone. Are quarks fundamental particles, or made of something smaller yet? Electrons are believed to be fundamental, but you wouldn't want to bet your life on it.

Still, theoretical physicists crave simplicity. They'd like to have a model of reality that snaps together neatly. Their standard model, developed in the 1960s and 1970s, is widely viewed as unwieldy, like a contraption with too many loose ends and knobs and dangling bits. It includes 57 fundamental particles, with no rhyme or reason to many of the numbers describing how the particles interact. "We had a theory that started out really beautiful and elegant," says Joe Lykken, a theorist at Fermilab, "and then someone beat on it and made it really ugly."

The standard model can't explain several towering mysteries about the universe that have their roots in the minuscule world of particles and forces. If there's one truly extraordinary concept to emerge from the past century of inquiry, it's that the cosmos we see was once smaller than an atom. This is why particle physicists talk about cosmology and cosmologists talk about particle physics: Our existence, our entire universe, emerged from things that happened at the smallest imaginable scale. The big bang theory tells us that the known universe once had no dimensions at all—no up or down, no left or right, no passage of time, and laws of physics beyond our vision.

How does an infinitely dense universe become a vast and spacious one? And how is it filled with matter? In theory, as the early universe expanded, energy should have condensed into equal amounts of matter and antimatter, which would then have annihilated each other on contact, reverting to pure energy. On paper, the universe should be empty. But it's full of stars and planets and charming French villages and so on. The LHC experiments may help physicists understand our good fortune to be in a universe that grew with just enough more matter than antimatter.

What about the riddle of dark matter? Scrutiny of the motion of distant galaxies indicates that they are subject to more gravity than their visible matter could possibly account for. There must be some exotic hidden matter in the mix. A theory called supersymmetry could account for this: It states that every fundamental particle had a much more massive counterpart in the early universe. The electron might have had a hefty partner that physicists refer to as the selectron. The muon might have had the smuon. The quark might have had ... the squark. Many of those supersymmetric partners would have been unstable, but one kind may have been just stable enough to survive since the dawn of time. And those particles might, at this very second, be streaming through your body without interacting with your meat and bones. They might be dark matter.

By smashing pieces of matter together, creating energies and temperatures not seen since the universe's earliest moments, the LHC could reveal the particles and forces that wrote the rules for everything that followed. It could help answer one of the most basic questions for any sentient being in our universe: What is this place?

There's one puzzle piece in particular that physicists hope to pick out of the debris from the LHC's high-energy collisions. Some call it the God particle.

The first thing you learn when you ask scientists about the God particle is that it's bad form to call it that. The particle was named a few years back by Nobel Prize-winning physicist Leon Lederman, who has a knack for turning a phrase. Naturally the moniker took root among journalists, who know a good name for a particle when they hear one (it beats the heck out of the muon or the Z-boson).

The preferred name for the God particle among physicists is the Higgs boson, or the Higgs particle, or simply the Higgs, in honor of the University of Edinburgh physicist Peter Higgs, who proposed its existence more than 40 years ago. Most physicists believe that there must be a Higgs field that pervades all space; the Higgs particle would be the carrier of the field and would interact with other particles, sort of the way a Jedi knight in Star Wars is the carrier of the "force." The Higgs is a crucial part of the standard model of particle physics—but no one's ever found it.

Theoretical physicist John Ellis is one of the CERN scientists searching for the Higgs. He works amid totemic stacks of scientific papers that seem to defy the normal laws of gravity. He has long, gray hair and a long, white beard and, with all due respect, looks as if he belongs on a mountaintop in Tibet. Ellis explains that the Higgs field, in theory, is what gives fundamental particles mass. He offers an analogy: Different fundamental particles, he says, are like a crowd of people running through mud. Some particles, like quarks, have big boots that get covered with lots of mud; others, like electrons, have little shoes that barely gather any mud at all. Photons don't wear shoes—they just glide over the top of the mud without picking any up. And the Higgs field is the mud.

The Higgs boson is presumed to be massive compared with most subatomic particles. It might have 100 to 200 times the mass of a proton. That's why you need a huge collider to produce a Higgs—the more energy in the collision, the more massive the particles in the debris. But a jumbo particle like the Higgs would also be, like all oversize particles, unstable. It's not the kind of particle that sticks around in a manner that we can detect—in a fraction of a fraction of a fraction of a second it will decay into other particles. What the LHC can do is create a tiny, compact wad of energy from which a Higgs might spark into existence long enough and vivaciously enough to be recognized. Building a contraption like the LHC to find the Higgs is a bit like embarking on a career as a stand-up comic with the hope that at some point in your career you'll happen to blurt out a joke that's not only side-splittingly funny but also a palindrome.

You can take an elevator down into the LHC tunnel if you wear a hard hat and carry an emergency oxygen mask. When I visited, I found a major construction project still under way, with all the usual sounds of blowtorches and metal saws. Workers were installing magnets. They've since completed the process, having installed more than 1,600 magnets, most half the length of a basketball court and weighing more than 30 tons.

Oddly enough, none of those magnets will accelerate particles. The acceleration will come from electrical waves in a separate apparatus that boosts particles around the ring. The job of the magnets is to nudge the beams of particles to bend ever so slightly around the ring. Lots of particles moving at nearly the speed of light have only one desire in life: to keep moving straight ahead. So the bend needs to be gradual—thus the 17-mile circumference of the ring.

When the particles collide, they'll produce showers of debris as their energy gets transformed into mass. The physicists won't see the Higgs itself in that shower, but two of the four major experiments that the LHC will perform are capable of recording the detritus of the disintegrating Higgs—the telltale signal that a Higgs is decaying. And the assumption is that only the rare collision—one among many trillions—will produce a Higgs. Most collisions won't result in anything terribly interesting. The particle—or rather its debris—will show up in a detector's computers, found by sorting through massive amounts of data measured in petabytes—thousands of trillions of bits.

A major issue for CERN is how to decide that they've found the Higgs. How much proof do you need? They've got two experiments competing to find the same particle. Do they announce the discovery by one experiment even if the other hasn't confirmed it yet?

The relationship between the ATLAS and CMS experiments is like Coke versus Pepsi. They're working the same side of the street, but with different techniques. And they're highly competitive. The day I went to see ATLAS, the man in charge, Peter Jenni, found out that I'd already seen the CMS experiment. "Now you'll see something bigger," he said. His voice carried a slight my-detector-is-better-than-yours tone.

CMS was built at the surface and will be lowered in several large chunks down through a shaft into a cavern along the tunnel. Tactlessly, I asked Dave Barney, one of the CMS scientists, what would happen if something went wrong and a part was dropped. You know, splat.

"That won't happen," he said fiercely. "That's the worst thing imaginable." I realized that I was treading on delicate territory whenever I asked what kinds of things could go wrong with the LHC. No, the collider can't blow up the world, but this is high- energy physics. When those magnets are turned on, scientist Richard Jacobsson pointed out, a person swinging a hammer in the vicinity would do well to wear a helmet.

When the LHC starts smashing particles, Europe will suddenly become the dominant location for particle physics, and the United States will find itself struggling to figure out how to stay relevant. Perhaps that's a petty concern given the magnitude of what the LHC might turn up, but it's something people talk about. Since the Manhattan Project there's been a general notion that the U.S. dominates the world of physics. Until now, the energy frontier has been at Fermilab, home of the Tevatron. That collider has found some important particles, but it might not have quite enough juice to nail the Higgs.

Some U.S. money has gone into the LHC, which will cost billions of dollars: five, maybe ten—the exact number is elusive (the science will be precise, but the accounting apparently follows the Uncertainty Principle). But most of the engineering is being done by European firms. Jürgen Schukraft, who supervises an LHC experiment named ALICE (which will re-create conditions the same as those just after the big bang), said, "The brain drain that used to go from Europe to the States definitely has reversed."

The cynic might say that there's no practical use for any of this, that there might be other uses for all the money and brainpower going into these particle guns. But we live in a civilization shaped by physics. We know that the forces within an atom are so powerful that, unleashed and directed against humanity, they can obliterate cities in an instant. The laptop computer on which I'm writing uses microprocessors that would not exist had we not discovered quantum physics and the quirky behavior of electrons. This story will be posted on the World Wide Web—invented, in case you hadn't heard, at CERN, by computer scientist Tim Berners-Lee. Maybe you're reading it while listening to your iPod, which wouldn't exist but for something called "giant magnetoresistance." Two physicists discovered it independently in the late 1980s, with not much thought of how it might eventually be used. It became crucial to making tiny consumer electronics that used magnetized hard disks. The physicists won a Nobel Prize in 2007, and you got a nifty sound system that's smaller than a Hershey bar.

When I asked Peter Jenni why the LHC is important, he said, "Humankind differs from a collection of ants. We have intellectual curiosity; we need to understand the mechanisms of life and the universe."

And anyone who thinks these big machines are soulless contraptions should listen to Richard Jacobsson. The LHC is replacing a particle detector he worked with for a decade. He came to know every inch of that instrument. He understood its moods and idiosyncrasies. The day the engineers came to rip it out, Jacobsson was overcome with emotion. "I had tears in my eyes," he said. "When they cut the cables, I thought blood would flow out." Now entire lives are wrapped up in the new machine, which physicists have been dreaming about since the 1980s.

Many people at CERN are hoping they'll get more than just answers: They'd like to uncover some new mysteries. John Ellis confided that he wouldn't even mind if the LHC failed to find a Higgs. "Many of us theorists would find that failure much more interesting than if we just find another boring old particle that some theorists predicted 45 years ago."

New puzzles seem a sure bet. After all, the universe doesn't seem to be constructed for our investigative convenience. We're big, sloppy meat-creatures who haven't even taken a good census of the species of bacteria that live in our bodies. One day I asked George Smoot, a Nobel laureate physicist, if he thinks our most basic questions will ever be answered.

"It depends on how I'm feeling on any particular day," he said. "But every day I go to work I'm making a bet that the universe is simple, symmetric, and aesthetically pleasing—a universe that we humans, with our limited perspective, will someday understand."