In an observatory unlike any other in the world, deep beneath China’s Guangdong Province, the world’s largest neutrino detector has begun operations. The 20,000-ton liquid scintillator detector at China’s Jiangmen Underground Neutrino Observatory (JUNO) has now completed filling, according to a statement from the Chinese Academy of Sciences Headquarters. Now, with JUNO’s official launch, a new era of physics experiments involving mysterious “ghost particles” known as neutrinos—and very large ones—is about to take shape within the gigantic spherical observatory. At the heart of the state-of-the-art JUNO facility is a massive 20,000-ton liquid-scintillator detector, which rests at the center of an equally large, 44-meter-deep pool of water. A longtime reporter on science, defense, and technology with a focus on space and astronomy, he can be reached at micah@thedebrief.org.