• China retired one neutrino detector and is now building its replacement.
• These detectors are huge, relatively inexpensive, and filled with water and sensors.
• Neutrinos also come from the Sun and stars, but scientists continue to try and isolate Earthly neutrinos.

A new facility in China will soon begin hunting for ghostly neutrino particles in a spherical particle jail that’s nearly half a mile beneath a granite mountain. In order to detect neutrinos, the key is to reduce other noise as much as possible, and that is often done by burying the detector—whether under the ocean or in deep underground facilities like those at CERN. The new device replaces the recently decommissioned Daya Bay facility that previously led China’s neutrino studies.

Neutrino detectors are built deep underground or underwater because scientists want to shield them as much as possible from cosmic sources of neutrinos. Neutrinos can pass through the Earth itself, and they leave a distinctive signature because of how they interact with other particles. As neutrinos rip through, they ionize, or charge, atoms and molecules. This releases electrons that float freely, and these are measured. Neutrinos themselves have no charge, and while they’re extremely common in our universe, it’s just difficult to isolate their influence if scientists keep their open ears held to all of outer space.

The Jiangmen Underground Neutrino Observatory (JUNO) detector is a giant, transparent acrylic sphere that will hold a reservoir of water or solvents. The sphere is nearly 120 feet in diameter, and JUNO leaders say it will be the largest clear detector of its kind in the world. The sphere is lined with sensors that detect any minor perturbation of the liquid inside, because it’s these micro-scale movements that hold evidence of passing neutrinos. Scientists will access the facility through an underground tunnel in the bedrock.

Neutrinos were theorized nearly 100 years ago as a product of nuclear reactions throughout the universe. That means our nuclear power plants generate them, in the same way that the Sun and other stars do. Previous work has shown that these neutrinos can’t be stopped, and because they can be detected by “plain water,” researchers have suggested using neutrino detectors to make sure world powers are adhering to nuclear nonproliferation agreements. Plain water can be augmented with other materials to make these detectors even more effective.

China leads the world in building new nuclear fission power plants, and many are mega plants with multiple enormous reactors that all emit neutrinos. There’s a huge split in China between its densely populated eastern portion, which inclu