Deep beneath the mountains of southern China, a machine unlike anything else on Earth has been awakened. After more than a decade of excavation, construction, and precision assembly, the Jiangmen Underground Neutrino Observatory, known simply as JUNO, has officially begun its hunt for the most elusive particles in the cosmos. On August 26, 2025, the collaboration announced that the detector had entered full operation. With its activation, a new front has opened in one of the most intense global races in modern science: the quest to understand neutrinos, particles that pass through every person on Earth by the trillions every second yet remain almost impossible to capture.
What has been built is staggering in scale. At the heart of JUNO is a transparent acrylic sphere more than 35 meters across, suspended inside a cavern the size of a skyscraper. This sphere holds 20,000 tons of a liquid scintillator, an engineered fluid so pure that it can register the tiniest spark of light created by a neutrino collision. Around it, more than 40,000 photomultiplier tubes line the walls, waiting in darkness for a faint glimmer to appear. The sphere itself sits inside a water pool 44 meters deep, which provides shielding from natural background radiation. The entire system is buried 700 meters below ground in Guangdong Province, encased in rock to block out cosmic interference.
The challenge is simple to describe but almost impossible to solve: neutrinos interact so rarely with matter that detectors must be unimaginably large and extraordinarily sensitive just to record a handful of events. They are called ghost particles for good reason. A neutrino can travel through light years of solid lead without being stopped. To observe even a few, scientists need massive detectors, isolation from background noise, and a steady source of neutrinos.
JUNO’s location was chosen for one key reason. Just over 50 kilometers away stand two of China’s most powerful nuclear power stations, Taishan and Yangjiang. Nuclear plants are prolific producers of antineutrinos, the antimatter counterpart to neutrinos. Every second, billions of them stream outward in all directions. Some of those travel through the Earth and into JUNO. When one happens to collide with a proton in the scintillator, a tiny flash of light is produced. That flash is caught by the surrounding tubes, converted into an electrical pulse, and recorded by supercomputers in the control room. From these tiny flashes, physicists hope to uncover answers to questions that have defied decades of research.
The most immediate goal is to settle the mystery of neutrino mass ordering. We know there are three flavors of neutrinos: electron, muon, and tau. We know they can oscillate, flipping from one flavor to another as they travel. We know they have mass, because oscillations cannot happen otherwise. What no one has been able to determine is which of the three corresponding mass states is the lightest and which is the heaviest. This is known as the neutrino hierarchy problem. In the normal ordering, two lighter states sit below a heavier one. In the inverted ordering, one lighter state sits beneath two heavier ones. Distinguishing between these possibilities requires measuring subtle distortions in the energy spectrum of reactor neutrinos. JUNO was designed precisely for this task, with an unprecedented energy resolution of about 3 percent at 1 MeV.
This question is not academic. The mass ordering matters for understanding how neutrinos shaped the early universe, how galaxies formed, and why matter dominates over antimatter in our present reality. It ties directly into physics beyond the Standard Model, potentially opening doors to new theories that explain dark matter, baryogenesis, and the asymmetry between matter and antimatter. The stakes are as high as they come in fundamental physics.
JUNO’s journey began almost two decades ago. The concept was proposed in 2008, approved by the Chinese Academy of Sciences in 2013, and excavation began in 2015. Carving out the underground hall was an engineering challenge, requiring the removal of hundreds of thousands of cubic meters of rock. The acrylic sphere was assembled piece by piece in a carefully controlled environment to avoid distortions. By 2023, photomultiplier tubes had been installed across the entire interior, each calibrated to catch even the faintest light. In December 2024, engineers began filling the detector with scintillator and ultrapure water, a process that took months. Finally, in August 2025, JUNO was declared operational.
Even before the official start date, signals were recorded. On August 24, two days before launch, antineutrino events were registered that matched predictions. These early results confirmed that JUNO was not only functioning as intended but also surpassing expectations. For the hundreds of scientists who had dedicated years of their careers to the project, those first signals were proof that the enormous gamble of scale and precision had paid off.
The collaboration behind JUNO is international in scope. Nearly 700 scientists from 17 countries are part of the effort. The Institute of High Energy Physics in Beijing leads the project, but contributions have come from laboratories across Europe, the United States, and Asia. This reflects the global importance of the neutrino problem and the recognition that no single nation or institution could take on such a challenge alone. Yet, there is also competition. Japan’s Hyper-Kamiokande, a massive water Cherenkov detector now under construction, aims to explore neutrinos from accelerators and the atmosphere. The United States is building the Deep Underground Neutrino Experiment in South Dakota, a facility that will study a controlled neutrino beam fired from Illinois. JUNO, however, is first to activate, giving China a head start in what has become a global race.
The scale of JUNO’s ambition goes beyond mass ordering. If a supernova occurs in our galaxy during the next three decades, JUNO will be able to record the burst of neutrinos that arrive seconds before the light does. That information would give astrophysicists a real-time look at the processes inside a collapsing star, something no telescope could achieve. JUNO will also track solar neutrinos produced in the heart of the Sun, adding precision to models of nuclear fusion. Atmospheric neutrinos, generated when cosmic rays strike Earth’s atmosphere, will provide further opportunities to test oscillation behavior at different energies.
The sensitivity of the detector opens doors to discoveries that could completely alter physics. One possibility is the detection of sterile neutrinos, a proposed fourth type of neutrino that would interact even less than the known three. Another is proton decay, a process predicted by many grand unified theories in which protons are not ultimately stable but can disintegrate over immense timescales. Finding either would force a rewrite of the Standard Model. While such detections are uncertain, JUNO will be one of the few facilities on Earth capable of searching for them.
The engineering triumph of JUNO is itself a story of extremes. The acrylic sphere had to be built to withstand the pressure of thousands of tons of liquid without warping. The scintillator had to be purified to levels far beyond industrial standards, since even trace contaminants would drown out the weak neutrino signals. The photomultiplier tubes, some 20 inches across, are so sensitive that they can detect a single photon. Each is calibrated to function for decades in water without degradation. To manage the flood of data, JUNO’s computing systems rely on advanced algorithms and distributed processing, with machine learning methods expected to assist in distinguishing true neutrino signals from background noise.
From a financial perspective, JUNO represents an investment of more than 300 million dollars. From a scientific perspective, its potential value is immeasurable. If it settles the mass ordering problem, it will immediately reshape theoretical models. If it detects a galactic supernova, it will provide data that cannot be obtained anywhere else. If it uncovers hints of new particles or processes, it will change the trajectory of physics for generations.
The launch of JUNO also reflects China’s broader ambition to become a leader in fundamental science. Hosting the largest neutrino detector of its kind positions China at the center of particle physics, a role traditionally dominated by facilities in the United States, Europe, and Japan. With JUNO’s success, China has demonstrated its ability not only to fund but also to deliver a project of unprecedented complexity. That achievement has strategic as well as scientific consequences, signaling China’s arrival as a full participant in the highest levels of international research.
As the detector begins its three-decade run, the scientific community will be watching closely. Within a few years, JUNO is expected to publish results that directly address the mass ordering problem. As data accumulates, more precise measurements of oscillation parameters will follow. If unexpected anomalies appear, the consequences could be revolutionary. For now, what is certain is that JUNO has begun, the signals are real, and the hunt for ghost particles has entered a new era with a machine buried in the mountains of Guangdong.
The story is not one of quiet science but of a colossal gamble that has just begun to pay off. A giant sphere glowing in total darkness, surrounded by an ocean of water, waits for sparks that may arrive only once in billions of tries. Every second, trillions of neutrinos pass through us unseen. At JUNO, each of those passing ghosts is a potential key to the structure of the universe. The machine has opened its eyes, and for the first time on this scale, humanity is staring back into the silent flow of particles that have crossed the cosmos since the beginning of time.
