Neutrinos (see below) have long puzzled physicists around the world as one of the biggest mysteries in the science community. Often known as the "ghosts" of the universe, huge numbers of them zip around space as they pass through practically everything without causing a single reaction. Though they seem weightless, scientists in Japan discovered evidence that neutrinos have mass.
Everything is made up of basic building blocks known as elementary particles. They hold the key to the origins of the universe, but there's still plenty we don't know about them.
Space is awash in neutrinos, which are one of the elementary particles - over 10 billion of them pass through an area the size of one printed letter in just a single second.
Neutrinos are, however, invisible to the naked eye. The particles simply sail through everything, whether that be newspapers, buildings or Earth. Physicists have long believed these ghostly particles had no mass.
An answer to the mystery would eventually be found roughly 1,000 meters underground in the old Kamioka mine in Hida, Gifu Prefecture. There, future Nobel laureate Masatoshi Koshiba and his collaborators had built Kamiokande, a neutrino observatory.
Spewed out from supernovas as well as the sun, neutrinos are also constantly being formed when cosmic rays collide with atmospheric elements.
The phenomenon was being observed at Kamiokande, which stored about 3,000 tons of purified water in a tank. Muon neutrinos and electron neutrinos occasionally collided with the water, manifesting in a brief flash of light.
In 1986, then University of Tokyo teaching assistant Takaaki Kajita was wringing his neck trying to make sense of the data. Neutrinos were forming the way they would anywhere else - but fewer muon neutrinos were being detected from underground, or the other side of Earth, compared to those from above Kamiokande or the skies above Japan.
What was the difference between Japan's upper atmosphere and the other side of Earth?
Neutrinos that formed in the atmosphere on the other side of Earth had to travel long distances to reach Kamiokande.
"What if they were transforming into tau neutrinos along the way, which can't be picked up by Kamiokande?" Kajita thought at the time.
There was a theory that neutrinos were able to morph into different types - but to do so would mean they had mass. To claim neutrinos have mass would contradict what was deemed common sense among particle physicists at the time.
Kajita, who is now director of the Institute for Cosmic Ray Research, needed more data to support his theory, which he gathered by running Super Kamiokande and its roughly 50,000-ton water tank for a year starting 1996.
The particle physicist presented his research proving that neutrinos can indeed transform, at an international academic conference in Takayama, Gifu Prefecture, in June 1998. He received a standing ovation for opening a new chapter for neutrino research spearheaded by Japan.
Subatomic particles that get their name from having a neutral charge. Thought to not bond with each other; first detected in experiments that used a nuclear reactor in 1956. Comes in three "flavors" of muons, electrons and tau.
Mysteries of the universe: Where is all the antimatter?
How was the universe created? How did it come to be the way it is today? Neutrinos could hold the answers to such questions that have puzzled physicists since the particle was first discovered.
In the 20th century, a draft of a paper breaking down the characteristics of elementary particles assumed neutrinos had zero mass.
The premise spawned several theories on the origin of the universe - but neutrinos were found to have mass, spurring research from an entirely new perspective.
One particularly vexing mystery is the missing presence of antimatter, which have opposite charge and other particle properties as matter, in the universe. Neutrinos also have their equivalent antimatter counterpart.
There should have been equal amounts of matter and antimatter when the universe was created - so where have they gone? Did they disappear?
Prof. Takashi Kobayashi, who works at the High Energy Accelerator Research Organisation, believes that by "studying the differences in the way neutrinos and anti-neutrinos transform, we may be able to grasp some kind of clue."
Unlocking the mysterious properties of neutrinos could bring scientists another step closer to solving the antimatter riddle.
Plans are also under way to design the Hyper-Kamiokande, boasting more than 20 times the capacity of its predecessor Super-Kamiokande.
The flagship observatory will be built in Hida, Gifu Prefecture, with a goal to start experiments in 2025.
Tucked away deep in the mountains, another revolutionary physics discovery could be born.