SUDBURY, Ontario - Penn physicist Josh Klein still gets a chuckle out of the incredulous look from a border agent as the carful of scientists crossed into Canada.

"We told him we needed to go to the bottom of a mine - to look for particles from the sun," Klein said.

The 34-year-old assistant professor at the University of Pennsylvania has sacrificed much of his time over the last three years searching for neutrinos deep underground. Neutrinos are among the few indivisible building blocks of the universe, along with quarks, electrons, and more exotic particles called taus and muons. Far smaller than atoms, these fundamental particles are believed to make up virtually everything - the modern equivalents of the ancient Greeks' air, earth, fire and water.

Many of the properties of neutrinos are still unknown. What researchers here and elsewhere find out about them will determine the answers to deep questions about the nature of the universe. Scientists around the world are looking to the results of this experiment to potentially steer physics in new directions.

Of the known fundamental entities, the neutrinos are the most ethereal, and also the most elusive. "Because they are so hard to study, they are the most mysterious particle we have," Klein said. They are also the most abundant: 100 billion neutrinos zoom through a spot the size of your thumbnail every second. In fact, they actually do zoom through your thumbnail. At night, neutrinos from the sun stream through the Earth and up through your floor. Their tendency to ignore matter is what makes them so hard to detect.

That's why the scientists must go to the bottom of a mine to find them. The thousands of feet of rock above their sophisticated neutrino-detecting experiment act as a screen, blocking most other possible species of particle except neutrinos, of which a tiny fraction will leave a trace.

Ontario had just the kind of mine they needed: a 94-year-old nickel mine in the town of Sudbury, a four-hour drive into the lake country northwest of Toronto.

At least there's no traffic on his commute to work, Klein noted wryly as he drove a rental car among the the slag heaps, smokestacks, and scrawny white birch trees that represent part of Sudbury's attempt to repair an environment ruined by decades of mining and smelting operations. In the town's worst days, he said, acid rain had so denuded the landscape that NASA did experiments with moon walking here.

For Klein, this place has become a home away from the Center City apartment that he shares with his wife, a music historian. But working here is an opportunity that many other young physicists envy, because this neutrino-catcher, the Sudbury Neutrino Observatory, is considered one of the most exciting projects in particle physics.

The first major results will be announced this afternoon at a conference in Canada and seminars at Penn and several other U.S. institutions that took part in the 100-person effort. Penn's contingent was headed by physicist Eugene Beier.

The name neutrino, for "little neutral one," was coined by Italian-born physicist Enrico Fermi. The particle's existence was first postulated in the 1930s, when physicists noted a shortage of energy coming from certain nuclear reactions - as if some unknown particle were carrying it away. Real evidence of neutrinos' existence wasn't found until the 1950s, through an experiment using a nuclear reactor, which also generates neutrinos. The researchers won a Nobel Prize.

The current effort in Canada is an attempt to solve a 35-year-old mystery involving neutrinos from the sun, where they play a role in the reactions that produce heat and light.

"I can see the great designer designing the sun and realizing that, without neutrinos, it wouldn't work," joked Leon Lederman, a physicist at the Fermi National Accelerator Laboratory near Chicago, who won his own Nobel for demonstrating that neutrinos come in more than one type.

The mystery is that some of the sun's neutrinos are disappearing.

In the early 1960s, scientists took on the audacious task of calculating the rate at which neutrinos should emanate from the sun, and then tried to verify this by catching a sample at the bottom of a gold mine in South Dakota. They got about a third of the number they expected, leaving the other two-thirds missing.

The favored explanation for the shortfall, odd as it sounded, was that the three types of neutrinos may be able to switch identities. The sun emits only one type, called the electron neutrino (so named because it's usually produced alongside an electron), the only type that was "visible" to these early detectors. Perhaps, the thinking went, some are switching into the harder-to-detect kinds - the so-called muon neutrino or tau neutrino.

The nature of this switching behavior would depend on other unknown properties of neutrinos, including what mass they have. And these properties figure into some exotic theories about the cosmos - postulating, for example, that there are higher dimensions of space beyond length, width and depth. So the results of experiments like this could help point our understanding of nature in new directions.

Because so many neutrinos permeate the universe, even if they have a tiny mass they may have also played a role in shaping galaxies after the Big Bang.

"What's really neat about neutrino physics is it has a finger into so many areas of physics," Janet Conrad, a physicist at Columbia University, said.

On a gray morning in early May, Klein pulled into the entrance to the Creighton Mine, where big men in coveralls are still extracting nickel ore. The mine has the advantage of depth - it plunges more than a mile beneath the surface, where only neutrinos are likely to go on their own. Neutrino-detecting experiments are, as a general rule, gigantic tanks of some liquid, so to start off, the physicists needed a gigantic cavern.

In 1990, INCO, the company that owns this mine, blasted out a barrel-shaped chamber 70 feet across and 100 feet high in an unused part of the labyrinth.

Once the blasting was complete, in 1993, the scientists built their neutrino detector, a spherical container 40 feet in diameter with a long neck, like a giant chemistry flask, filled with 1,000 tons of heavy water - water in which the hydrogen atoms of H2O are replaced with their heavy version, deuterium. The $75 million detector finally came on line in 1998.

To get to the work area, Klein must don a miner's outfit, hard hat, safety glasses, heavy boots, and a thick canvas belt to hold the five-pound battery for his headlamp. This is all required gear for entering the mine. Similarly outfitted miners come and go, some of them giving him a curious look.

"You can wear the belt around your waist," Klein explained, adding that the miners consider it "cooler" to wear it slung low over the hip. Being part of the neutrino team, he laughed, "already makes you uncool to the miners."

Once equipped, Klein steps into a metal cage for an ear-popping 15 m.p.h. descent to one of the lowest of the mine's dozen-odd levels, opening into a dark, gritty tunnel 11/4 miles below. The walls are covered in a kind of chain mail with bolts the size of dinner plates. Rock this far down is under intense pressure. It could burst out of the wall and crush everyone down here, but the bolts somehow manage to hold it all in.

"I used to think that what miners did was look for ore," Klein said, "but what they really spend most of their time doing is keeping the mine from killing them."

It takes a mile-and-a-half hike down the dim, puddled corridor, past various wrong turns and dead ends, to reach the entrance to the experiment.

Studying the properties of neutrinos - specifically, trying to determine whether the three kinds switch identities - is much harder than just detecting one. Scientists need to pull in hundreds of them in order to make statistically significant observations.

A trillion trillion neutrinos pass through the underground tank every day, but only 10 or so of these strike one of the deuterium atoms in the heavy water in just the right way to send out a shower of other particles, the end result of which is a "hit" in some of the surrounding detector panels.

Unfortunately, lots of other things can set off the detectors as well, including the traces of radiation present in almost everything - dust, clothes, people, water. Since the mine is filthy, keeping the dirt out requires an elaborate ritual in which everyone showers and puts on bright-blue bunny suits, hair nets, and dust-free boots.

Clean and outfitted, Klein enters the inner laboratory, whose bright lights and cleanly coated walls give it a surreal look, like some secret lab in a spy movie.

Equipment whirs and buzzes all around the lab, getting louder above the experiment, where a maze of pipes circulates the water. The water itself is worth about $350 million because it takes a tremendous effort to concentrate the naturally occurring traces of heavy water from an ordinary lake. Luckily, Atomic Energy of Canada Ltd. agreed to lend it to the researchers, expecting it back at the end of the experiment.

Klein has spent many a graveyard shift over the last three years down here, in what they call the control room. Computer screens flash with numbers, indicating that the detector has caught something. As with scientists in many other disciplines, however, most of Klein's time will be spent searching but not finding: Only a few of the signals will come from neutrinos, and those will be extraordinarily difficult to distinguish from the inevitable imposters.

Three years ago, the Penn scientists and their collaborators lost their chance to be the first to confirm the neutrino-identity-switching theory, called neutrino oscillations. That honor went to a Japanese experiment that saw evidence of this behavior among neutrinos that came from a different source: cosmic rays in outer space.

Still, the Sudbury apparatus has capabilities beyond the Japanese setup. It is the only one in the world that can tell not only that neutrinos disappeared but where they went or what they became. That's because it can run in one mode that detects only electron neutrinos, and in another mode that picks up all three types, though without distinguishing among them. Comparing the numbers found in the two modes will determine whether some of the electron neutrinos are indeed switching to muon or tau neutrinos.

The equipment, of course, does not actually see neutrinos of any type. It merely detects pulses of light - the end product of the showers of particles released when a neutrino strikes the nucleus of a deuterium atom and breaks it apart.

Today's announcement could be one of several possible findings. The scientists could demonstrate that neutrinos from the sun do switch identities, which would be big news. The researchers could also determine that neutrinos turn into a new, fourth type of neutrino, which would be an even bigger deal - the first new particle to be discovered in years.

Or they could say they need more information.

In any case, the team plans to continue gathering data for a more definitive announcement, perhaps in the fall. So the days of neutrino mining are far from over.