In the dark silence three kilometers beneath the Mediterranean Sea, a scientific machine called KM3NeT was slowly awakening. It had been built to capture ghostly messengers from deep space — particles so elusive that most would slip through the entire planet without a trace. But on February 13, 2023, something remarkable happened. A particle slammed into the water near the still-growing KM3NeT detector, triggering a signal so intense it lit up a third of the sensors.

When Paschal Coyle, a physicist at the Centre for Particle Physics of Marseille, first tried to analyze it, his computer couldn’t take it. “When I first tried looking at this event, my program crashed,” he told New Scientist. What Coyle and his colleagues had found was not just another neutrino. It was the most energetic neutrino ever observed, clocking in at an estimated 220 peta-electronvolts (PeV), an energy 16,000 times greater than anything created at the Large Hadron Collider.

And it had traveled from somewhere far beyond our galaxy.

A New Window to the Violent Universe

Neutrinos are among the strangest particles in nature. First predicted in 1930 by Wolfgang Pauli and first detected in 1956, they carry no electric charge and have nearly no mass. They barely interact with matter. “They are special cosmic messengers that reveal the secrets of the most energetic phenomena in the universe,” said Rosa Coniglione, a deputy spokesperson for KM3NeT at the time of the discovery in the Mediterranean.

Normally, detecting a neutrino requires immense, creative experiments. KM3NeT is one of the boldest yet. Its giant arrays of spherical detectors, strung like beads on vertical lines anchored to the seafloor, use the Mediterranean’s clear waters as a detection medium. When a neutrino finally smashes into an atom, it can create a muon — a heavier cousin of the electron — that speeds through the water faster than light can travel there. This produces a cone of bluish Cherenkov light, like a sonic boom but made of photons, which the detectors can pick up.

Despite being only a tenth complete, KM3NeT managed to capture the neutrino now known as KM3-230213A. “This first ever detection of a neutrino of hundreds of PeV opens a new chapter in neutrino astronomy,” said Paschal Coyle.

The find stunned researchers because nothing like it had ever been seen before. IceCube, the South Pole’s massive neutrino observatory, had previously detected cosmic neutrinos up to about 6 PeV. But this? This was on another scale entirely.

“It was really surprising,” said Rosa Coniglione. “How does a smaller detector that’s been turned on for a shorter period of time see the rarest of them all, the highest-energy neutrino?” wondered Naoko Kurahashi Neilson, a neutrino astronomer at Drexel University.

Where Did It Come From?

That is now the million — or perhaps trillion — dollar question.

One possibility is that the neutrino was born in a cosmic particle accelerator like a blazar — a galaxy with a supermassive black hole at its heart firing jets of particles directly toward Earth. IceCube had previously traced a lower-energy neutrino to such a blazar in 2018. Yet, when KM3NeT researchers searched the sky region from which the 220 PeV neutrino arrived, they found no obvious source.

“They did not identify any convincing source,” said Shirley Li, a physicist at the University of California, Irvine.

Another tantalizing idea is that the neutrino may be a cosmogenic neutrino. These are ultra-rare particles created when high-energy cosmic rays — protons and atomic nuclei flying across the universe at nearly the speed of light — slam into the cosmic microwave background radiation, the afterglow of the Big Bang.

“It is a very exciting possibility,” said Li. Scientists have predicted the existence of cosmogenic neutrinos for decades, but no experiment had confirmed them. If KM3NeT’s neutrino is cosmogenic, it would be the first direct evidence that these highest-energy neutrinos truly exist.

Yet there is a catch. If cosmogenic neutrinos are out there, why hasn’t IceCube already seen them? “It’s challenging to say this event is from a cosmogenic flux,” admitted Li.

For now, the origin remains unknown. As Yuri Kovalev from the Max Planck Institute for Radio Astronomy put it, “By adding observations from other telescopes, we seek to connect the acceleration of cosmic rays, the production of neutrinos, and the role of supermassive black holes in shaping these energetic phenomena.”

Amazing and Mysterious

Whether the neutrino came from a hidden blazar, a cosmic collision billions of light-years away, or something even stranger, the implications are profound. Neutrinos offer a pristine view of the universe’s most violent processes. Unlike charged cosmic rays, neutrinos travel in straight lines, completely unaffected by magnetic fields. They also pass through gas, dust, and light without getting deflected or absorbed.

“There are huge implications for science and astronomy,” said Kurahashi Neilson. Neutrinos could, for instance, reveal how black holes grow and how stars explode. They might even explain why the universe is made of matter instead of antimatter. Some physicists suspect neutrinos may be key to uncovering physics beyond the Standard Model, the ruling theory of particles and forces.

For instance, if neutrinos are Majorana particles — particles that are their own antiparticles — it could explain why the Big Bang produced more matter than antimatter. “If neutrinos are their own antiparticle, that might explain where all the antimatter in the early universe went,” said Ryan Nichol of University College London.

And the cosmos itself could become a new kind of laboratory. “This is a whole new arena in which to look for deviations,” said Li.

KM3NeT is still under construction. Once completed around 2029, it will fill more than one cubic kilometer of deep-sea volume with 230 detection lines, each studded with optical modules. Scientists also have their eyes on other projects, like the Pacific Ocean Neutrino Experiment off Canada’s coast, and continued work at IceCube’s upgraded observatory.

Until then, researchers will wait for more ghostly signals from the abyss—tiny flashes of light that carry stories from the most extreme corners of the universe.

As Coyle put it, “This event shows their detector works beautifully. There’s so much more you can do with two detectors versus one. We are moving towards ultra-high-energy neutrino astronomy.”

And with that, a new era of cosmic discovery begins.

The findings were reported in the journal Nature.