In February 2023, a detector 3.5 kilometres beneath the Mediterranean caught the aftermath of the most energetic neutrino ever seen — a 220-petaelectronvolt ghost particle, roughly 16,000 times beyond CERN’s strongest collisions, with no confirmed source anywhere in the sky.

In February 2023, a single particle crossed a detector resting on the floor of the Mediterranean, and it carried more energy than any neutrino ever recorded. Two years of careful analysis later, the team behind the detector put a number on it: around 220 petaelectronvolts, roughly 16,000 times the energy of the most violent collisions produced at CERN’s Large Hadron Collider.

It is the most energetic neutrino ever seen.

No one knows where it came from.

What was caught, and where

The detector is KM3NeT, or more precisely its ARCA array, a forest of light sensors strung along cables and anchored about 3.5 kilometres down off the coast of Sicily. Neutrinos almost never interact with anything, which is why they are called ghost particles, but very occasionally one strikes an atom in the seawater and throws off a charged particle that leaves a faint cone of blue light. The sensors sit in the dark and watch for it.

On 13 February 2023, a single event lit up more than a third of the active sensors. It was one muon, ploughing almost horizontally through the entire detector, the track of a neutrino with an extraordinary amount of energy behind it. The event was logged as KM3-230213A, and the collaboration reported it in Nature in February 2025.

How much energy that is

The estimated energy, about 220 petaelectronvolts, is hard to picture, so the comparison with the Large Hadron Collider helps. The LHC accelerates protons to around 13 trillion electronvolts before smashing them together, and it is the most powerful accelerator ever built. This neutrino arrived carrying something like 16,000 times that.

The figure is an estimate, with real uncertainty, worked out from how much light the muon produced. Even at the low end of the range, though, it is far beyond anything seen before, and it is the first direct evidence that neutrinos reach these energies in nature at all.

The awkward part

There is a complication, and an honest account has to include it. KM3NeT is new and still incomplete. Two other experiments, the much larger IceCube array buried in the Antarctic ice and the Pierre Auger Observatory in Argentina, have watched the same energy range for far longer, and neither has recorded an event like this.

That a smaller, younger detector caught one first is either luck or a hint that something does not quite add up. A single event cannot tell the two apart.

It will take more of them.

Where it might have come from

The source is the real mystery. Tracing the muon’s path back across the sky turned up no confirmed object, only candidates. Broadly, there are two kinds of answer.

One is that it came from an extreme astrophysical accelerator, something like the jet of a supermassive black hole, flinging particles out at energies no laboratory can reach. The other is that it is cosmogenic, produced not by a single object but when an ultra-high-energy cosmic ray, crossing intergalactic space, collides with the faint leftover light of the Big Bang. In that case the neutrino is a by-product of the universe at large, with no home address to be found.

Both possibilities remain open, and neither has been confirmed.

What to watch

KM3NeT is still being assembled, and as more sensor strings go in, its reach will grow. The question that matters now is plain: whether it, or IceCube, catches a second neutrino like this one.

If a few more turn up, the first will mark the opening of a new way of seeing the high-energy universe. If none do, it will stay what it is today, a single, genuine, unexplained particle, recorded one February morning under three and a half kilometres of seawater.