Emerge's 2025 Project of the Year: The Deep-Sea Machine That Caught an Ultra High-Energy Ghost
SOURCE:Decrypt|BY:Josh Quittner
Beneath the Mediterranean, physicists at the Cubic Kilometre Neutrino Telescope Initiative built a cathedral of glass spheres that listens for cosmic whispers—and this year, it heard the most energetic neutrino humanity has ever recorded.
In brief
The KM3NeT project is redefining astronomy by pairing deep-sea engineering with multi-messenger physics long before construction is even finished.
KM3NeT captured a 220-PeV neutrino in 2023—an energy so extreme it reshaped expectations for cosmic particle accelerators.
Its ORCA and ARCA detectors turn a cubic kilometer of Mediterranean water into a precision observatory for the universe’s most elusive messengers.
The Mediterranean Sea, usually celebrated for its sun-drenched coasts and azure surface, hides a secret in its crushing darkness.
Three and a half kilometers beneath the waves off the coast of Sicily, the water is pitch black, near freezing, and under pressure intense enough to crumple a submarine as if it were an empty beer can. It's a place of profound silence, undisturbed by the chaotic affairs of the surface world. Yet, in this abyss, something is watching.
Thousands of glass spheres, strung up like massive pearls on vertical cables rising from the seafloor, hang in the darkness. They’re listening for the universe to whisper its secrets.
On a quiet Tuesday in February 2023, the silence was broken by a phantom flash of blue light that lasted mere nanoseconds. It was a signal that had traveled billions of light-years, passing through galaxies, stars, and the entire mass of the Earth before ending its journey here, in the sensors of a machine that wasn't even fully built yet.
That flash was the footprint of a neutrino carrying 220 Peta-electronvolts (PeV) of energy, a number so large it borders on the absurd for a single subatomic particle. It was the highest-energy neutrino ever detected by humanity, a messenger from a cosmic cataclysm of unfathomable power.
But the true marvel wasn't just the particle; it was the machine that caught it.
Why it matters
The editors of Decrypt's Emerge have selected the KM3NeT (Cubic Kilometre Neutrino Telescope) Initiative as the 2025 Project of the Year, because it represents a fundamental shift in our relationship with the cosmos.
While traditional astronomy has spent centuries refining how we look at the universe, KM3NeT allows us to sense its very core, detecting particles that pass through matter as if it weren't there. We chose this initiative not just for the historic confirmation of the 220 PeV event published this year, but for the sheer audacity of its engineering.
By turning the Mediterranean abyss into the world's largest high-energy physics laboratory, KM3NeT has proven that we can build precision instruments in the most hostile environments on Earth to answer the most elusive questions of the galaxy. It is a triumph of international cooperation, resilience, and vision, delivering world-changing science before construction is even complete.
Why is this machine necessary? First, one needs to understand the paradox of the neutrino. Often called "ghost particles," neutrinos are the second-most abundant particles in the universe, outnumbered only by photons of light.
They are produced by nuclear reactions—in the heart of our sun, in the explosion of dying stars, and in the violent jets of black holes. Trillions of them are passing through your body right now. You cannot feel them, nor do they feel you.
Neutrinos have almost no mass and no electric charge, meaning they do not interact with electromagnetic fields. While a photon of light can be stopped by a sheet of paper or a wall, a neutrino can pass through a block of lead a light-year thick without slowing down. This makes them the perfect cosmic messengers.
Unlike light, which can be blocked by dust clouds, or charged particles, which are bent by magnetic fields, neutrinos travel in straight lines from their source to us. If we can catch them, then we can point directly back to the engines of the universe—supernovae, blazars, and colliding neutron stars—and see exactly what is happening inside them.
But their greatest strength is also their greatest flaw: because they interact with nothing, they are nearly impossible to catch. To detect even a handful of them, you need a target of immense size—a "net" so large that purely by the laws of probability, a neutrino will eventually crash into an atom within it. You also need total darkness to see the faint spark that the collision produces. Building a detector of that size on land is prohibitively expensive and technically impossible.
So, the physicists of KM3NeT decided to borrow a detector that nature had already built: the ocean.
The underwater cathedral
The premise of KM3NeT is elegant in its simplicity but brutal in its execution. When a high-energy neutrino finally crashes into an atomic nucleus in the water, it obliterates the nucleus and creates a shower of secondary charged particles, such as muons.
These particles rocket through the water faster than light can travel in that same medium (though still slower than the speed of light in a vacuum). This breaking of the "light barrier" creates a shockwave of blue light known as Cherenkov radiation—essentially the optical equivalent of a sonic boom.
The KM3NeT infrastructure is designed to capture this fleeting blue glow. The "telescope" does not use lenses or mirrors. Instead, it consists of hundreds of vertical lines, or "strings," anchored to the sea floor and held taut by submerged buoys. Attached to these strings are the Digital Optical Modules (DOMs)—pressure-resistant glass spheres about 17 inches in diameter.
"The wonderful thing about a neutrino telescope is that we do not need to point it explicitly, it will catch neutrinos from all directions; the pointing is done in software," Paul DeJong, speaking on behalf of the project, told Decrypt.
DeJong, a professor at the University of Amsterdam and senior scientist at Nikhef (Dutch National Institute for Subatomic Physics), is known for his leadership roles in major collaborations like CERN's ATLAS experiment (Higgs boson discovery). He is also the designated spokesperson for the KM3NeT neutrino telescope project.
Inside each sphere is a marvel of miniaturization. While older neutrino detectors used single, large light sensors, KM3NeT’s DOMs contain 31 smaller photomultiplier tubes arranged like the compound eye of a fly. This multi-eye design gives them exceptional directional sensitivity and allows them to distinguish between a genuine neutrino signal and the background "noise" of bioluminescent sea creatures or radioactive potassium salts naturally present in seawater.
The scale is hard to visualize. The detector is not a single solid object but a sparse forest of sensors spread over a cubic kilometer of water. It is a cathedral built of nothing but cable, glass, and the sea itself—taller than the Burj Khalifa, yet completely invisible from the surface.
A tale of two telescopes
The initiative is actually two separate detectors, each tuned to a different frequency of the cosmic orchestra.
The first, located off the coast of Toulon, France, is called ORCA (Oscillation Research with Cosmics in the Abyss). Here, the sensors are packed tightly together. ORCA’s job is to catch lower-energy neutrinos that have traveled through the Earth from the other side.
By studying how these neutrinos change "flavors"—a quantum mechanical shape-shifting trick—as they pass through our planet's mantle, ORCA aims to solve the "mass hierarchy" problem: determining which of the three types of neutrinos is the heaviest. This sounds abstract, but the answer holds the key to understanding why the universe is made of matter rather than antimatter.
The second detector, and the site of the recent record-breaking discovery, is ARCA (Astroparticle Research with Cosmics in the Abyss). Located in the deeper waters off Capo Passero, Italy, ARCA is the giant. Its sensors are spaced widely apart to monitor a massive volume of water. ARCA is the true "telescope," designed to catch the ultra-high-energy monsters arriving from deep space.
The 220 PeV breakthrough
The scientific community was electrified earlier this year when the KM3NeT collaboration published their analysis of the event now known as KM3-230213A. To put 220 PeV into perspective, typical neutrinos from the sun arrive with energies in the range of Mega-electronvolts (MeV). A PeV is a billion times more energetic than that. The particle detected by ARCA carried as much kinetic energy as a professionally served tennis ball, all packed into a subatomic point smaller than an atom.
This detection confirmed what theorists had long suspected but could not prove: that the universe contains natural particle accelerators far more powerful than the Large Hadron Collider. While the Collider runs on kilometers of magnets and electricity, the sources of these neutrinos run on gravity and magnetic turbulence on a galactic scale.
The 220 PeV event likely originated from a blazar—a supermassive black hole shooting a jet of plasma directly toward Earth. The detection has effectively pushed the boundaries of the Standard Model of particle physics, challenging our understanding of how high energy can go before the laws of physics impose a speed limit.
Engineering the impossible
The success of KM3NeT is a victory for physics. Deploying these lines is a logistical ballet. Each string is wound into a compact spherical launcher frame, lowered to the seabed by a specialized vessel, and then acoustically triggered to unfurl, rising hundreds of meters into the water column.
The challenges are relentless. The pressure at these depths is 350 atmospheres. The saltwater is highly corrosive. The electronics must operate autonomously for decades without maintenance, as you cannot simply send a diver down to change a fuse. The team had to develop new fiber-optic data transmission systems to send terabytes of raw data from the sea floor to shore stations in real-time.
In early 2025, the ARCA site faced a power failure in its seafloor network—a setback that required a complex robotic intervention to fix. Despite these hurdles, the team remains undeterred.
"The technology is proven, but the detector is not finished," DeJong admitted. "At this time, about 25% of the envisaged detector elements have been deployed... but actually completing the detector will be significant work."
The timeline reflects the magnitude of the task, targeting 2030 for ORCA and 2031 for ARCA.
"Size matters for catching elusive neutrinos, so we need that extra volume," DeJong said. "The difficult conditions so deep in the sea remain challenging."
The new era of astronomy
As 2025 draws to a close, KM3NeT is still growing. New lines are being deployed in both France and Italy. But it has already fulfilled its promise. We have moved from an era of purely visual astronomy to "multi-messenger" astronomy. We can now watch a star explode with telescopes, feel the ripple in spacetime with gravitational wave detectors, and catch the ghost particles fleeing the scene with neutrino hunters.
"I would like to see neutrinos from sources that also emit other types of radiation, gamma rays for example, or gravitational waves," DeJong says, looking toward the future. "The combination of all information will really enable us to make progress in the understanding of the universe."
The KM3NeT Initiative reminds us that to see the furthest reaches of the heavens, sometimes we must look deep into the abyss. It also reminds us of our own intimate connection to those distant celestial events.
As DeJong notes: "We are literally stardust! Isn’t that a fantastic concept?"
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