In the early hours of February 13, 2023, the depths of the Mediterranean near Sicily were as calm as ever for the local marine life until a piercing blue light, invisible to the human eye, streaked through the darkness. This light marked the detection of the highest energy particle of its kind ever recorded.

This glow was the signature of a cosmic neutrino, an elusive and typically unobtrusive “ghost particle” so called because of its near-zero chance of interacting with normal matter that composes our world. A neutrino can travel through a light-year of lead without so much as a scratch. Each second, around 100 trillion of these particles, mostly emitted by the sun, pass through our bodies unnoticed. This makes them rare catches but invaluable carriers of insights into hidden cosmic phenomena within the dense cores of stars and galaxies.

This particular discovery was made by KM3NeT, a major European collaborative project that’s currently under construction. It plans to utilize roughly one cubic kilometer of the Mediterranean’s waters filled with instruments for its twin detectors. Despite being incomplete, this ambitious project has already yielded remarkable findings—a neutrino originating from outside our galaxy, possessing extraordinary energy.

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“This neutrino was found in a completely new energy range, 30 times higher than any neutrino previously observed,” said Paschal Coyle, a neutrino physicist at the French National Center for Scientific Research and a member of the KM3NeT team, during a recent press conference.

KM3NeT’s ARCA detector, situated nearly 3.5 kilometers beneath the sea surface off Sicily’s coast, is tailored to catch these astrophysical neutrinos. It does this by observing the aftermath of their rare interactions with water molecules.

“These neutrinos carry so much energy when they strike—it results in a massive explosion of particles,” explains Kate Scholberg, a physicist at Duke University not involved in the study. “Though they seldom interact, when they do, it’s a spectacular event, producing a brilliant display of particles. The light emitted during these interactions is what we detect.”

When this particular neutrino collided, it was during a time when ARCA was monitoring with only 21 of its planned 230 detection lines. The neutrino struck a water molecule outside the detector, generating a high-energy muon—a heavier cousin of the electron—which in turn produced a series of particles and a distinctive pale blue light known as Cherenkov radiation that swept across ARCA’s sensors. By analyzing this light, scientists were able to trace the muon’s underwater trajectory, calculate the energy of the original neutrino, and determine its likely point of origin in space.

The researchers estimate that the neutrino’s energy was around 220 peta electron volts, more than 30 times higher than the most energetic neutrino detected before. To illustrate this during the press conference, Aart Heijboer, a physicist at the Nikhef National Institute for Subatomic Physics in the Netherlands and co-author of the study, compared it to the energy of a Ping-Pong ball falling about one meter in Earth’s gravity. This neutrino contained that much energy in just a single subatomic particle, he explained. Or, to put it another way, “This is about 1,000 times more energetic than anything we could produce on Earth,” added Bryan Ramson, a neutrino physicist at the Fermi National Accelerator Laboratory in Illinois, who was not involved in the study.

While this detection is intriguing, it raises more questions than it answers. KM3NeT is joining a long-standing neutrino telescope called IceCube, which has been operational near the South Pole since 2011. IceCube was designed to capture these high-energy neutrinos just as effectively as KM3NeT, yet its current record-holding observation holds just one-thirtieth of the energy of KM3NeT’s recent discovery, puzzling some experts.

“My first reaction was astonishment. And how could this be possible without IceCube having detected something similar before?” inquired Ignacio Taboada, a physicist at the Georgia Institute of Technology and current spokesperson for the IceCube collaboration.

Moreover, the KM3NeT team couldn’t link their high-energy neutrino to a specific source. They scanned the small section of sky from which the neutrino likely originated but found no obvious culprits like a blazar—an active galactic nucleus often suspected in such cosmic events. This suggests the neutrino could have come from a high-speed cosmic ray colliding with a photon from the extragalactic background light or the cosmic microwave background, the team hypothesized.

“This event is peculiar; I think that’s a fair conclusion.”

The esoteric nature of astrophysical neutrinos makes them a challenging but fascinating subject for scientific study. Unlike most astronomical observations that rely on photons—which can be blocked or scattered—neutrinos travel in straight lines over vast distances without interference, providing a unique perspective on the universe that can extend back to its very beginnings. “With light, you can only see so far,” Ramson notes, referring to the photonic fog of the cosmic microwave background, emitted about 380,000 years after the Big Bang. “Neutrinos offer a way to peer beyond that veil, to see further back than ever before.”

Whether scientists are on the verge of uncovering these deep cosmic secrets hinges on whether KM3NeT continues to make groundbreaking observations like the 2023 detection and whether IceCube can replicate such findings after years without spotting such high-energy particles. The current discrepancy in detections is perplexing, to say the least. “They might have just gotten lucky; it’s hard to say,” Scholberg muses. “It’s very intriguing, and it definitely means we need more data.”

Taboada agrees that the discovery is tantalizing but emphasizes that more observations are necessary to understand what KM3NeT has unearthed. “If it turns out to be an astrophysical neutrino, that would be monumental,” he states. But he wants more evidence. “This event is peculiar; I think that’s a fair conclusion,” he observes. “It’s unexpected, no matter how you look at it.”