Physicists are putting the finishing touches on the Jiangmen Underground Neutrino Observatory, or JUNO, which will delve into the secretive ways of neutrinos, a family of extremely lightweight subatomic particles with no electric charge, and their antiparticle partners, antineutrinos.Set to start taking data in summer 2025, JUNO aims to determinewhich of the three types of neutrinos is heaviest. It will be the largest detector of its type in the world.At the heart of the detector, located 700 meters underground in China, sits a roughly 35-meter-wide acrylic sphere. It will be filled with 20,000 metric tons of liquid scintillator, which emits light in response to particles produced when a passing antineutrino interacts with a proton in the liquid. Tens of thousands of photomultiplier tubes will eye the scintillator for antineutrino-induced glimmers. Surrounding the sphere, water will fill a cylindrical pit to help filter out subatomic particles that are not antineutrinos but might mimic them. Scientists began filling this pit with water on December 18.Once operational, the detector will scrutinize antineutrinos released as plentiful by-products from two nuclear power plants, each about 50 kilometers away.The Jiangmen Underground Neutrino Observatory is nestled into the mountains of Kaiping, in southern China. The detector sits 700 meters below the surface. JADE GAO/AFP via Getty Images A stainless-steel structure, about 40 meters in diameter, supports the JUNO detector. IHEP JUNOs 35-meter acrylic sphere (seen here from underneath) will eventually hold 20,000 metric tons of liquid scintillator. IHEP Workers install photomultiplier tubes (gold) that will search for antineutrino-induced flashes of light from the liquid scintillator contained within the 35-meter acrylic sphere. JADE GAO/AFP via Getty Images Tens of thousands of photomultiplier tubes (gold) will surround the acrylic sphere (top). YueXiang Liu, IHEP