A Shield of Stone
Beneath Jinping Mountain in Sichuan, China, sits the world's deepest and largest underground research facility. The China Jinping Underground Laboratory (CJPL) is buried under 2,400 meters of solid marble, a rock with naturally low radioactivity. This massive rock overburden acts as a natural shield, reducing the flux of cosmic ray muons—high-energy particles from space—to less than 0.2 per square meter per day. This is a hundred-millionth of the rate on the Earth's surface, making CJPL the best-shielded laboratory on the planet.
The laboratory's existence resulted from a massive engineering project. It was constructed inside tunnels originally excavated for the Jinping-II Hydroelectric Dam. Physicists at Tsinghua University identified the site's potential in 2008, and the first phase of the lab (CJPL-I), with a volume of 4,000 cubic meters, was inaugurated on December 12, 2010. An advantage of this location is its horizontal access; unlike labs in old mines that require elevators, heavy equipment can be driven directly into CJPL through a 17.5 km long tunnel.
In December 2023, a major expansion called the Deep Underground and Ultra-low Radiation Background Facility (DURF), or CJPL-II, became operational. This second phase increased the total laboratory volume to an enormous 330,000 cubic meters, making it the largest facility of its kind.
The Dark Matter Hunt
The extreme quiet of CJPL's shielded environment allows scientists to listen for some of the faintest signals in the universe. The primary research goal is the direct detection of dark matter, the substance that accounts for about 85% of all matter but does not interact with light. Two major experiments lead this search: PandaX and CDEX.
The PandaX (Particle and Astrophysical Xenon) experiment, operated by a collaboration led by Shanghai Jiao Tong University, uses liquid xenon as its detection medium. Its current iteration, PandaX-4T, contains 3.7 tonnes of xenon in its sensitive detector. When a hypothetical dark matter particle—a Weakly Interacting Massive Particle (WIMP)—strikes a xenon nucleus, it should produce a tiny flash of light and a small electrical charge, which sensitive instruments can record. The dual-phase (liquid and gas) nature of the detector allows for precise 3D reconstruction of the event's location, helping to distinguish potential signals from background radiation.
The China Dark Matter Experiment (CDEX), led by Tsinghua University, takes a different approach. It uses p-type point-contact germanium (PPCGe) detectors. These detectors are cooled to extremely low temperatures and can detect very small energy depositions, making them particularly sensitive to low-mass WIMPs. The laboratory also hosts experiments on neutrinos and nuclear astrophysics, such as the Jinping Neutrino Experiment (JNE) and the Jinping Underground Experiment for Nuclear Astrophysics (JUNA).