The idea that the ancient, legendary, fishing, and sailing Mediterranean Sea now conceals, somewhere beneath its dark surface, a machine designed to address the most fundamental question science has ever asked: why is there something rather than nothing? There is something subtly remarkable about this idea. A kilometer or so below the surface of the ocean, vertical strings of basketball-sized glass spheres extend upward into the chilly water like an unearthly forest. Europe’s deep-sea neutrino telescope, KM3NeT, is listening for signals from the violent periphery of the universe.
It hunts particles known as neutrinos, which are nearly impossible to identify. Every second, billions of them flow through every human body without leaving any trace. There is no electric charge, very little mass (at least a million times lighter than an electron), and nearly complete disregard for regular matter. Since the 1930s, physicists have been pursuing them experimentally after learning about them theoretically. According to Paschal Coyle of the French National Centre for Scientific Research, who oversees European support for the KM3NeT infrastructure, “neutrinos are the most interesting particles around right now.” “Among the fundamental particles, they are the least understood.” What makes them so fascinating is their combination of being extremely abundant and almost impossible to study.
The indirectness with which KM3NeT captures them is almost poetic. A neutrino will occasionally, very infrequently, collide with an atomic nucleus, producing a short-lived cascade of secondary particles. A faint blue glow known as Cherenkov radiation is released when a collision occurs in a transparent material, such as water. It is fleeting. The ultra-sensitive optical sensors in KM3NeT are made specifically to capture that flash. It’s similar to looking for a single struck match in a dark room the size of a city. It feels amazing that it functions at all. Super-Kamiokande in Japan and IceCube in Antarctica, which is buried in polar ice, both employ similar strategies. However, KM3NeT has a unique advantage: the ocean itself protects the detector from the continuous noise of other cosmic radiation, and the deep Mediterranean provides exceptional water clarity.
Then came the morning of February 13, 2023, and what remains a mystery to scientists. KM3NeT detected a neutrino with an energy charge of 220 petaelectronvolts, designated KM3-230213A. That is thirty times more energetic than any neutrino that has ever been seen, and until you sit with the number for a moment, it is nearly meaningless. “We weren’t really expecting to find such an event,” Coyle acknowledged. “Many simulations had to be redone.It is still genuinely unclear where it originated. Blazars, which are far-off galaxies with supermassive black holes shooting energy jets straight toward Earth, are one popular theory.
According to a different theory, it originated when photons moving across the universe collided with high-energy cosmic rays, creating what are known as cosmogenic neutrinos. Regarding the third choice, Coyle is truthful. “Or we were just lucky,” he remarked. “It could be that KM3NeT managed to spot a rare, very high-energy neutrino by chance.” A clearer directional trace should help identify the source in the upcoming months as measurements become more precise. However, it’s still a mystery, and the field is humbly humming.
KM3NeT is made up of two installations that collaborate with one another. Located off the coast of Sicily, ARCA is designed to track high-energy neutrinos coming from deep space. The more subtle focus of ORCA, located close to Toulon, France, is the study of how neutrinos change between their three known forms, or flavors, as they move. The ordering of neutrino masses can be revealed by these flavor changes, or oscillations, and that is very important. Neutrino mass ordering is still missing from the current Standard Model of physics, which describes all known fundamental particles. Filling it might alter our comprehension of the entire theory. “All the experiments that try to measure the difference between a neutrino and an anti-neutrino get confused because they don’t know what the mass ordering is,” Coyle said. “It’s an important input to figuring out why there’s more matter than antimatter.”
The deepest question is the last one. Equal amounts of matter and antimatter should have destroyed one another after the Big Bang, which occurred about 13.7 billion years ago, leaving the universe as nothing but empty space. Matter made it out alive. We are real. Neutrinos are one of the most plausible explanations for how physics tipped the scales. Every aspect of fundamental physics would be affected if they proved to be their own antiparticles, which is still a possibility.
Senior physicist Aart Heijboer of the Dutch National Institute for Subatomic Physics, who contributed to the telescope’s design, remembers being fascinated by the idea’s peculiarity. “It seemed like a crazy idea to build a detector at the bottom of the sea to catch these very weird particles,” he replied. “That caught my imagination.” When you listen to the people who created it, you get the impression that part of its appeal was its audacity—the understanding that asking extreme questions necessitates extreme infrastructure. There are currently over a thousand sensor modules in use. By 2027, six thousand are anticipated, making the current array much larger than it is now. The likelihood that another remarkable signal will emerge, flash momentarily in the dark water, and convey, if scientists can decipher it, a message about pre-Earth events increases with each new string of sensors added to the seafloor.
It is difficult to ignore the fact that Europe’s investment in this field is motivated by both practicality and scientific ambition. As early as 2006, the EU supported a design study, and ongoing support has made an idea that once seemed unrealistic a practical tool. The telescope that captured KM3-230213A is already among the most sensitive ears ever placed to the cosmos, regardless of whether it originated from a blazar, a cosmogenic collision, or something no one has yet figured out. It might still take years to find the answers. However, the sensors remain vigilant beneath the Mediterranean, and the sea maintains its age-old tranquility.
