A huge pool of clear liquid is hidden deep underground. Inside, there is nothing but a pristine 'broth' that flashes with faint light when an invisible particle flies through. This is the JUNO detector—20,000 tons of liquid scintillator built to hunt for neutrinos.
The main enemy of such a hunt is not darkness but its own noise. The liquid itself, the tank walls, even the bolts and gaskets can slightly 'glow'—emitting particles due to natural radioactivity. To distinguish signal from noise, scientists sifted through thousands of materials, like choosing the quietest instruments for an orchestra. In the end, in the central region of the detector with a radius of 17.2 meters, they achieved a count of only about 7 events per second at energies above 0.7 MeV. That's like hearing a whisper across a noisy square.
Construction of the detector and filling it with liquid were completed in August 2025. The first data already show that the natural radioactivity inside the active volume meets strict requirements. Now this giant underground 'eye' is ready to watch for elusive particles that will tell us how matter is put together. And again, the silence of the underground pool, but now you can make out the faintest flashes.
The liquid scintillator in JUNO is so transparent that light from a flash can travel tens of meters through it before being captured by sensitive photodetectors.