Fifty-three nuclear reactors surround it. They stand at power stations scattered across the islands of Japan, each one silently broadcasting a stream of electron antineutrinos -- particles so ghostly that trillions pass through your body every second without touching a single atom. Deep beneath the mountains of Hida, Gifu Prefecture, a machine called KamLAND waits to catch the handful that do not pass through. The Kamioka Liquid Scintillator Antineutrino Detector sits inside an old drift mine shaft in the Japanese Alps, at the Kamioka Observatory -- a scientific complex operated by the University of Tokyo and run by a team from Tohoku University. What KamLAND has found there, a thousand meters underground, has rewritten the physics of the smallest particles in the universe.
The heart of KamLAND is a nylon balloon thirteen meters across, filled with 1,000 metric tons of liquid scintillator -- a carefully blended mixture of mineral oil, benzene, and fluorescent chemicals. When an antineutrino from a distant reactor strikes a proton inside this liquid, it produces a positron and a neutron through a process called inverse beta decay. The positron emits a flash of scintillation light almost instantly; the neutron is captured by a hydrogen atom roughly 200 microseconds later, producing a second, characteristic gamma ray. This one-two punch of light -- the delayed coincidence signature -- is KamLAND's fingerprint for identifying antineutrinos against a vast background of other particles. Surrounding the balloon sits an 18-meter stainless steel containment vessel lined with 1,879 photomultiplier tubes, each one sensitive enough to detect a single photon. Beyond that, a 3.2-kiloton cylindrical water Cherenkov detector acts as a shield against cosmic rays and radioactivity seeping from the surrounding rock.
KamLAND began collecting data on January 17, 2002, positioned at an average flux-weighted distance of approximately 180 kilometers from Japan's commercial nuclear reactors. The physics was straightforward: count the antineutrinos arriving from those reactors and compare the number to what the reactors should be producing. If neutrinos have mass and can oscillate between different types -- or "flavors" -- some would transform during the 180-kilometer journey into forms that KamLAND could not detect. They would, in effect, disappear. And disappear they did. In the first 145 days of data, the detector observed only 54 antineutrino events where the no-oscillation model predicted far more. A longer 515-day run confirmed it: 258 events observed against 365 predicted. KamLAND had delivered definitive proof that antineutrinos oscillate, providing the most precise measurement of a key oscillation parameter and helping to resolve the decades-old solar neutrino problem.
In 2005, KamLAND turned its attention downward. The same detection technology that catches reactor antineutrinos can also detect geoneutrinos -- antineutrinos produced by the radioactive decay of thorium and uranium in the Earth's crust and mantle. These particles carry information about the heat engine driving plate tectonics, volcanic activity, and the planet's magnetic field. KamLAND detected a small number of geoneutrinos and used them to constrain the total radiogenic heat production of the Earth. By 2013, benefiting from reduced reactor backgrounds after Japanese nuclear shutdowns following the 2011 Fukushima disaster, KamLAND measured uranium and thorium radiogenic heat production at approximately 11.2 terawatts -- a result consistent with the reference Earth model and a direct window into the composition of the bulk silicate Earth. No drill hole has ever reached the mantle. KamLAND reads it with neutrinos.
Beginning in 2011, a new experiment called KamLAND-Zen suspended a balloon containing 320 kilograms of dissolved xenon-136 at the center of the detector. The goal: to observe neutrinoless double-beta decay, a hypothetical process that would occur only if the neutrino is its own antiparticle -- a so-called Majorana particle. Such a discovery would be among the most profound in physics, explaining why the universe contains matter rather than nothing. The first apparatus, KamLAND-Zen 400, ran through two phases from 2011 to 2015, setting increasingly stringent limits on the decay's half-life. In 2018, a larger balloon holding 750 kilograms of xenon was installed as KamLAND-Zen 800, and it began taking data in January 2019. By 2022, the collaboration had pushed the lower bound for the neutrinoless double-beta decay half-life past 2.3 times 10 to the 26th years, constraining the effective Majorana neutrino mass to between 36 and 156 millielectronvolts. The search continues, with plans for an even more sensitive successor detector called KamLAND2-Zen.
From the air, there is nothing to see. The mountains above Hida show only forested ridges and narrow river valleys -- no dome, no dish, no visible structure at all. The entire Kamioka Observatory complex is buried beneath a kilometer of rock, shielded from the cosmic ray bombardment that would overwhelm its sensitive instruments at the surface. The old Mozumi zinc mine, which once produced ore for industry, now produces data for fundamental physics. KamLAND shares this underground campus with its more famous neighbor, Super-Kamiokande, and together they have made the mountains of Gifu Prefecture one of the most important sites in particle physics on Earth. Every day, the liquid scintillator glows faintly with the arrival of particles from reactors, from the Sun, from the radioactive heart of the planet -- each flash a tiny message decoded in darkness.
The Kamioka Observatory (36.4225N, 137.3153E) is entirely underground beneath Mount Ikeno near Hida, Gifu Prefecture, and has no visible surface structures from the air. The surrounding terrain is mountainous with peaks exceeding 2,000 meters. The nearest airports are Toyama (RJNT) approximately 50 km to the north and Matsumoto (RJAF) approximately 80 km to the southeast. Look for the narrow Takahara River valley and the small city of Hida as landmarks. Exercise caution for mountain terrain and variable weather conditions typical of the Northern Alps.