LEAD, S.D. — Every morning, two dozen miners and engineers pack into a cage-like elevator for an 11-minute descent into the bowels of South Dakota’s Black Hills.
At the bottom, 1 mile beneath the surface, the cage door lifts and the workers file down a rocky, twisting corridor. At its end lies the result of three years of labor: two empty caverns, each as high as a seven-story building and so long that it takes a full second for your voice to reach the far wall and echo back.
For more than a century, these depths were scoured by miners in search of gold. Now they hold the Sanford Underground Research Facility, or SURF. In the coming years, some of the world’s top particle physicists plan to transform this realm into the listening end of an 800-mile, $5 billion tin-can telephone. With it, they hope to hear a whispered answer to an existential question: How did we get here?
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The message will be carried by incoming neutrinos — strange, elusive particles that weigh almost nothing and move almost as fast as light. At the telephone’s other end, a half-mile-wide particle accelerator operated by the Fermi National Accelerator Laboratory, just outside Chicago, will create trillions of neutrinos every second in a narrow beam pointed squarely at this cavern. They will sail underground, not through a tunnel but through three states’ worth of solid bedrock.
When the neutrinos arrive, physicists hope, they will finally explain how the Big Bang created ever so slightly more matter than its opposite, antimatter — an excess that constitutes everything in the universe today.
The telephone, officially called the Deep Underground Neutrino Experiment, or DUNE, is the largest science-engineering project beneath U.S. soil in history. It has taken a decade to reach this point and may need another decade before it begins its work. If all goes well, it will turn the squirrelly neutrino into a known quantity, filling a major gap in scientists’ understanding of the universe and, perhaps, return the United States to its former position at the center of particle physics.
What’s the Antimatter?
When an atom is split, its two fragments fly off at odd, oblique angles. In 1930, physicist Wolfgang Pauli dreamed up the neutrino to explain the behavior: There must be a third, unseen bullet that whizzes off in a third direction, as fast as light but ethereal as a specter, Pauli concluded.
It took decades for someone to prove him right. Neutrinos are the most numerous particles in the universe but the toughest to study; they evade particle detectors as easily as they pass through bedrock. They are so slippery that they are the only particles whose mass remains a total mystery.
Moreover, whereas every other particle has an immutable identity, neutrinos are shape-shifters. Once operational, the accelerator at Fermilab will produce one of the three “flavors” of neutrino. But by the time these reach South Dakota, some will have morphed into a different flavor.
“It’s as stark as if you turned into your grandmother as you walked to the kitchen, and then turned back into yourself as you walked back to your room,” Bryan Ramson, a physicist on the project, said from the DUNE control room in Batavia, Illinois. “That’s essentially what neutrinos do.”
Yet this “long-term, long-distance quantum effect,” he added, is how DUNE will use neutrinos to explain, well, everything.
The Cosmic Imbalance
Particle physicists hope that neutrinos can help solve a long-standing dilemma.
According to the best theories available, matter — everything we can see and feel in the universe — should not exist. Every particle of matter comes into being with a doppelgänger, a particle of antimatter (or “antiparticle”) with equal but opposite properties like charge and spin. Whenever a particle and its antiparticle meet, they annihilate each other. Particles and antiparticles can be made in equal measure, but they eventually find and destroy one another, leaving behind nothing.
The Big Bang somehow broke this rule. It created very slightly more matter than antimatter, and that sliver of matter composes everything we can see today. The shape-shifting properties of neutrinos, many physicists contend, may explain our cosmic genesis.
What Ramson and his DUNE colleagues are trying to learn is whether neutrinos shape-shift faster than antineutrinos do. Can neutrinos elude their doppelgängers by morphing into the wrong flavor, like fugitives donning a different costume? Might that be how the early universe ended up with slightly more matter than antimatter?
If DUNE detects a mismatch between neutrinos and antineutrinos, that could suggest that the hypothesis has merit. And because the neutrino is the only particle that physicists have not yet studied to death, it represents the last hope for resolving the existential conundrum.
“It’s the only open window into new frontiers,” said Jelena Maricic, a physicist at the University of Hawaii and a DUNE member.
Grand Ambitions
When the neutrinos from Fermilab reach the cavern in South Dakota, DUNE will have less than one-millionth of a second to trap and study them before they sail on through subterranean Wyoming and beyond.
The trap will consist of two enormous tanks, each filled with 17,000 metric tons of freezing-cold liquid argon; eventually, these tanks will be joined by two more, in an identical cavern down the hall. Once in a great while, an incoming neutrino will become corporeal and smash into an argon atom, generating a flash of light and a flicker of electricity. The detectors will measure these signals, offering physicists one more iota of information about neutrinos.
This rare event needs to occur many thousands of times for scientists to decipher whether neutrinos and antineutrinos behave differently. So the trap must be big, to stop as many neutrinos as possible. This called for excavating two caverns, and then filling them with thousands of tons of steel and some of the most sensitive electronics ever built, all carted down through the mine’s narrow esophagus. Mike Headly, the director of SURF, compared the construction to “building a ship inside a glass bottle, except the neck of the bottle is a mile long.”
Ramson said that “DUNE will be essentially the perfect long-baseline neutrino oscillation experiment.” He added, “If you gave me a trillion-dollar budget and all the time in the world, it’s hard to see how we would do better.”
The project’s grand ambition has brought grand challenges, not all of them foreseen.
The infrastructure of the mine shaft had to be overhauled before the lab could get the rocks out and the experiment in, delaying the excavation and costing at least $300 million. And Fermilab’s particle accelerator had to be upgraded — a billion-dollar expense — to deliver enough neutrinos to the detector. In 2021, the Department of Energy gave the lab a failing performance grade, and in 2023 it reopened the lab’s management contract to new bidders. Then, in May 2023, an iron worker fell 23 feet onto concrete and was severely injured. Work was halted for the remainder of the year, allowing the lab “to look at all our procedures and ensure the safety of our people,” Lia Merminga, Fermilab’s director, said.
The initial phase of the project — a first measurement using the two detectors in the first cavern — was originally estimated to be completed in 2035 at around $1.5 billion. It’s now scheduled for completion by 2040 at $3.3 billion. This does not include the billion-dollar accelerator upgrade or the two additional detectors that scientists hope to add, just the first of which will cost another $300 million. All told, the cost to American taxpayers for the entire undertaking could approach $5 billion.
Criticism in the scientific press has often been scathing, and it gave pause to some of the 1,400 scientists who had hitched their future to DUNE.
“This press, as it came out, was something I had to really think about as I was deciding what I wanted to do with the rest of my life,” Ramson said. “I’ve given up extremely lucrative careers elsewhere to follow my interests.” Were DUNE to be canceled, he said, “then I would have essentially made the wrong bet.”
Ron Ray, DUNE’s deputy project director, dismissed the critics. “Yeah there’s some noise out there, but the people who are writing those things don’t really know what they’re talking about,” he said. “The early days of a project are always defined by overwhelming optimism that never proves to be true.”
He argued that DUNE’s unexpected costs and delays were normal for a scientific endeavor of this size. He pointed to the James Webb Space Telescope, which was launched in December 2022 after years of delays and cost overruns and now regularly breaks cosmic ground. “There’s no remedy like success,” Ray said.
To Ramson, giving up on DUNE now “would signal to the world that America does not want to lead in particle physics anymore.” The lead instead would most likely go to Japan’s Hyper-K experiment, which is now under construction and scheduled to begin operating in 2027. Like DUNE, Hyper-K will use a newly upgraded particle accelerator to shoot neutrinos hundreds of miles to a newly excavated chamber in a mine. But it is simpler and sleeker, a modest step up from existing technology compared with DUNE. It is likely to progress faster and make the first, rough estimate of the imbalance between neutrinos and antineutrinos.
“Anything worth doing involves competition,” Sam Zeller, a physicist at Fermilab, said. But this isn’t a simple horse race, she added; neutrino experiments have always involved international collaboration. Besides, she said, DUNE has additional goals, which include looking for dark matter, the unseen substance that makes up most of the cosmos, and studying neutrinos from the cataclysmic deaths of faraway stars.
Behind the scenes, DUNE scientists said that they have made steady progress toward perfecting the liquid-argon detector, which was still a nascent technology in 2012 when DUNE’s designers gambled on it. “I look at the whole list of things that could have turned out differently and it’s like all the stars are aligning,” Zeller said.
A Turning Point
DUNE received a major morale boost in December. A panel of 32 prominent particle physicists, charged with ranking the field’s priorities for the coming decade, gave a top slot to the project’s completion.
“We felt the responsibility,” said Karsten Heeger, a physicist at Yale University and deputy chair of the panel, which is known as P5, for the Particle Physics Project Prioritization Panel. “It was a daunting, scary task.”
In the end, Heeger said, the scientific stakes were too high not to endorse the project. “This is a real opportunity for the U.S. to lead the world and to become the center of neutrino physics for the coming decades,” he said.
The report served as a mandate to “finish what we started,” Ray said. Merminga, Fermilab’s director, said, “I couldn’t be more pleased with the outcome.” She conceded that Fermilab had faced real challenges in recent years and added: “We’ve almost put it behind us.”
On Feb. 1, after more than a decade of planning and construction, the underground caverns were completed with a last blast of dynamite. The hole is there; now all the physicists — and the universe — must do is fill it.
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