Neutrinos are the oddballs of the subatomic particle family. They are everywhere, pouring in from the sun, deep space, and Earth and zipping through our bodies by the trillions every second. The particles are so tiny that they seldom interact with anything, making them extremely elusive and hard to study. Moreover, though neutrinos come in different types, or flavors, they can switch from one type to another as they travel near the speed of light. These weird behaviors, scientists believe, might point toward insights about the history of the universe and the future of physics.

After nearly six years of excavation, a gigantic neutrino laboratory is taking shape in the rolling hills of southern China, about 150 kilometers west of Hong Kong. The Jiangmen Underground Neutrino Observatory (JUNO) will be one of the world’s most powerful neutrino experiments, along with the Hyper-Kamiokande (Hyper-K) in Japan and the Deep Underground Neutrino Experiment (DUNE) in the U.S. Using two nearby nuclear power plants as neutrino sources, JUNO will aim to learn more about these particles and answer a fundamental question: How do the masses of the three known types of neutrinos compare to one another? Though researchers know the particles have a small amount of mass, the exact amount is unknown. Existing evidence shows that two of the flavors are close in mass and that the third one is different. But scientists do not know if that third type is heavier or lighter than the others: the former scenario is called the “normal mass ordering,” and the latter is named the “inverted mass ordering.”

The mass ordering of the neutrino is a key parameter for researchers to determine, says theoretical physicist Joseph Lykken of the Fermi National Accelerator Laboratory in Batavia, Ill. “In fact, all kinds of other things depend on the answer to that question,” he adds. For instance, the answer can help scientists better estimate the total mass of neutrinos in the universe and determine how they have influenced the formation of the cosmos and the distribution of galaxies. Even though neutrinos are the lightest of all known matter particles, there are so many of them in space that they must have had a big effect on the way ordinary matter is distributed. Understanding how neutrino masses are ordered could also help explain why the particles have mass at all, which contradicts earlier predictions.

More than 650 scientists, nearly half of whom are outside China, have been working on JUNO, which was first proposed in 2008. Later this year or in early 2021 researchers will start assembling the experiment’s 13-story-tall spherical detector. Inside, it will be covered by a total of 43,000 light-detecting phototubes and filled with 20,000 metric tons of specially formulated liquid. At 700 meters below the ground, once in a blue moon, an electron antineutrino (the specific type of particle that is produced by a nuclear reactor) will bump into a proton and trigger a reaction in the liquid, which will result in two flashes of light less than a millisecond apart. “This little ‘coincidence’ will count as a reactor neutrino signal,” says particle physicist Juan Pedro Ochoa-Ricoux of the University of California, Irvine, who co-leads one of the two phototube systems for JUNO.

As neutrinos arrive at the detector from the nuclear power plants 53 kilometers away, only about 30 percent of them will remain in their original identity. The rest will have switched to other flavors, according to Jun Cao, a deputy spokesperson for JUNO at the Institute of High Energy Physics (IHEP) at the Chinese Academy of Sciences, the project’s leading institution. The observatory will be able to measure this percentage with great precision.

Once operational, JUNO expects to see roughly 60 such signals a day. To have a statistically convincing answer to the mass ordering question, however, scientists need 100,000 signals—which means the experiment must run for years to find it. In the meantime JUNO will detect and study neutrinos from other sources, including anywhere between 10 and 1,000 of the particles from the sun per day and a sudden influx of thousands of them if a supernova explodes at a certain distance from Earth.

JUNO can also catch the so-called geoneutrinos from below Earth’s surface, where radioactive elements such as uranium 238 and thorium 232 go through natural decay. So far studying geoneutrinos is the only effective way to learn how much chemical energy is left down there to drive our planet, says geologist William McDonough of the University of Maryland, who has been involved in the experiment since its early days. “JUNO is a game changer in this regard,” he says. Though all the existing detectors in Japan, Europe and Canada combined can see about 20 events per year, JUNO alone should detect more than 400 geoneutrinos annually.

Right now the experiment is dealing with a flooding issue that has delayed the construction schedule by two years, says Yifang Wang, a JUNO spokesperson and director of IHEP. Engineers need to pump out 12,000 metric tons of underground water every day, but the water level has dropped significantly. It is not uncommon to run into flooding issues while building underground labs—an issue also experienced by the Sudbury Neutrino Observatory in Ontario. Wang believes that the problem will be solved before construction is completed.

JUNO should be up and running by late 2022 or early 2023, Wang says. Toward to end of this decade, it will be joined by DUNE and Hyper-K. Using accelerator-based neutrinos, DUNE will be able to measure the particle’s mass ordering with the greatest precision. It will also study a crucial parameter called CP violation, a measure of how differently neutrinos act from their antimatter counterparts. This measurement could reveal whether the tiny particles are part of the reason the majority of the universe is made of matter. “JUNO’s result on the neutrino mass ordering will help DUNE make the best possible discovery and measurement of CP violation,” Lykken says. The former experiment, along with the other neutrino observatories in development, could also reveal something scientists have not predicted. The history of neutrino studies shows that these particles often behave unexpectedly, Lykken says. “I suspect that the combination of these experiments is going to produce surprises,” he adds.

Editor’s Note (9/24/20): This story has been edited after posting to correct the figure for the amount of water engineers must pump out of the experiment site daily. It has also been updated to specify the distance between the two power plants and the experiment’s detector.