The world's largest transparent spherical detector began operation in China on Tuesday, making it the world's first operational ultra-large scientific facility dedicated to neutrino research with ultra-high precision.

Having completed the filling of its 20,000-tonne liquid scintillator detector, the Jiangmen Underground Neutrino Observatory (JUNO) in south China's Guangdong Province began taking data after more than a decade of preparation and construction.

The initial data taken during a trial operation showed that key performance indicators met or exceeded design expectations. This success positions JUNO to address one of the major questions in particle physics this decade: the ordering of neutrino masses.

"Completing the filling of the JUNO detector and starting data taking mark a historic milestone. For the first time, we have put into operation a detector of this scale and precision dedicated to neutrinos. JUNO will allow us to answer fundamental questions about the nature of matter and the universe," said Wang Yifang, JUNO's spokesperson and a researcher at the Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences (CAS).

Since neutrinos rarely interact with ordinary matter, they can easily zip through our bodies, buildings, or even the entire Earth without being felt, earning them the nickname "ghost particles." Due to their elusive nature, neutrinos are the least understood fundamental particles, requiring massive detectors to capture their faintest traces.

When passing through the detector, neutrinos have a small chance of bumping into the hydrogen nuclei in the liquid, triggering extremely faint flashes, which can be detected by the surrounded photo-multiplier tubes (PMTs) and then converted into electrical signals.

Located 700 meters underground near Jiangmen in Guangdong Province, JUNO detects antineutrinos produced by the Taishan and Yangjiang nuclear power plants both 53 kilometers away and measures their energy spectrum with record precision. Unlike other approaches, JUNO's determination of the mass ordering is independent of matter effects in the Earth and largely free of degeneracies with other neutrino oscillation parameters.

JUNO will also enable cutting-edge studies of neutrinos from the Sun, supernovae, the atmosphere and Earth. It will open new windows to explore unknown physics, including searches for sterile neutrinos and proton decay, according to the IHEP.

Proposed in 2008 and approved by the CAS and Guangdong Province in 2013, JUNO began underground construction in 2015. Detector installation started in December 2021 and was completed in December 2024, followed by the procedure of filling it with ultra-pure water and liquid scintillator.

At the heart of JUNO is a liquid-scintillator detector with an unprecedented mass of 20,000 tonnes, housed at the center of a 44-meter-deep water pool. A 41.1-meter-diameter stainless steel truss supports the 35.4-meter-diameter acrylic sphere, the liquid scintillator, over 45,000 PMTs and many other key components such as cables, magnetic shielding coils and light baffles.

"Building JUNO has been a journey of extraordinary challenges. It demanded not only new ideas and technologies, but also years of careful planning, testing, and perseverance," said Ma Xiaoyan, JUNO's chief engineer.

"Meeting the stringent requirements of purity, stability, and safety called for the dedication of hundreds of engineers and technicians. Their teamwork and integrity have turned a bold design into a functioning detector, which is now ready to open a new window in the neutrino world," she added.

JUNO is hosted by the IHEP and involves more than 700 researchers from 74 institutions across 17 countries and regions, according to the IHEP.

"The landmark achievement that we announce today is also a result of the fruitful international cooperation ensured by many research groups outside China, bringing to JUNO their expertise from previous liquid scintillator set-ups," said Gioacchino Ranucci, a professor at the University of Milano and Italy's national nuclear physics institute.

"The worldwide liquid-scintillator community has pushed the technology to its ultimate frontier, opening the path towards the ambitious physics goals of the experiment", added Ranucci, also JUNO's deputy spokesperson.

According to the IHEP, JUNO is designed to have a scientific lifetime of up to 30 years. It can be upgraded into a world-leading research facility that probes the absolute neutrino mass scale and tests whether neutrinos are Majorana particles--particles that are identical to their own antiparticles. It will address fundamental questions across particle physics, astrophysics, and cosmology, profoundly shaping our understanding of the universe.