KM3NeT, a cubic‑kilometre neutrino telescope built in the deep Mediterranean, was named Emerge’s 2025 Project of the Year after ARCA recorded a record‑breaking ~220 PeV neutrino (event KM3‑230213A) in February 2023. The array uses thousands of 17‑inch Digital Optical Modules, each with 31 photomultipliers, to capture Cherenkov light from neutrino interactions in seawater. Composed of two arrays — ORCA (Toulon) for oscillation and mass‑ordering studies and ARCA (Capo Passero) for ultra‑high‑energy astronomy — KM3NeT is ~25% deployed and aims to complete ORCA by 2030 and ARCA by 2031.
KM3NeT: The Deep-Sea Telescope That Caught a 220 PeV Neutrino — Emerge’s 2025 Project of the Year
The Mediterranean—famous for sunlit shores and clear blue water—hides a forbidding darkness beneath its surface. About 3.5 kilometres below the waves off Sicily, temperatures drop near freezing, light vanishes, and pressure rises to levels that would crush a submarine like a tin can. In that remote stillness, an observatory listens.
Hundreds of vertical cables rise from the seabed, studded with thousands of pressure-resistant glass spheres that hang like oversized pearls in the black. Their job: to register the faintest signals the universe sends.
On a quiet Tuesday in February 2023, that hush was broken by a blue flash lasting only nanoseconds. The signal had traversed billions of light-years, passed through galaxies and stars, and even through the bulk of the Earth before registering in an installation that was not yet finished.
That flash marked the passage of a neutrino carrying roughly 220 Peta‑electronvolts (PeV) of energy — the highest-energy neutrino recorded by humanity to date. The particle itself is extraordinary; the instrument that caught it is equally remarkable.
Why KM3NeT Matters
The KM3NeT (Cubic Kilometre Neutrino Telescope) Initiative has been named Emerge’s 2025 Project of the Year for changing how we probe the cosmos. Traditional telescopes collect light; KM3NeT collects nearly non-interacting particles — neutrinos — that travel in straight lines from the most violent engines in the universe. That makes them unique messengers for pinpointing sources such as blazars, supernovae and colliding neutron stars.
How the Detector Works
Neutrinos interact so weakly with matter that enormous detection volumes are required. KM3NeT uses the Mediterranean itself as that volume. When a very high-energy neutrino strikes a nucleus in the water, it produces cascades of charged particles (for example, muons) that move faster than light does in water, generating a faint cone of blue Cherenkov radiation.
Rather than mirrors or lenses, KM3NeT uses a sparse three-dimensional array of vertical "strings" anchored to the seafloor and kept vertical by submerged buoys. Each string carries Digital Optical Modules (DOMs): 17‑inch pressure-resistant glass spheres. Instead of one large sensor, each DOM contains 31 smaller photomultiplier tubes arranged like a compound eye, improving directional sensitivity and helping reject background from bioluminescence or natural radioactivity.
Two Complementary Arrays: ORCA and ARCA
The project actually comprises two detectors optimized for different science. ORCA (Oscillation Research with Cosmics in the Abyss), off Toulon, France, has a dense sensor spacing to study lower-energy neutrinos that traverse the Earth and to measure the neutrino mass ordering. ARCA (Astroparticle Research with Cosmics in the Abyss), off Capo Passero, Italy, spans a much larger volume with wider spacing to catch ultra-high-energy neutrinos arriving from space.
KM3-230213A — The 220 PeV Event
The KM3NeT collaboration published the analysis of an event dubbed KM3-230213A, recorded by ARCA in February 2023. At ~220 PeV, the detected neutrino carried energy comparable to a professionally served tennis ball, concentrated into a subatomic particle. The detection supports long-standing theoretical ideas that natural cosmic accelerators can vastly exceed the energies produced by human machines like the Large Hadron Collider. The event is consistent with an origin in a blazar — a supermassive black hole launching a relativistic jet toward Earth.
Engineering at Extreme Depths
Deploying and maintaining a kilometre-scale detector in waters at ~350 atmospheres of pressure requires new engineering solutions: launcher frames that unfurl strings from the seabed, robust multi-PMT DOMs, advanced fiber-optic data links to transmit terabytes of raw data in real time, and robotic repair capabilities. Early 2025 saw a seafloor power failure at the ARCA site that required complex robotic intervention; the team resolved it and continued deployment.
Paul DeJong, KM3NeT spokesperson: "The technology is proven, but the detector is not finished. About 25% of the planned detector elements have been deployed. Completing the array will be significant work — size matters for catching elusive neutrinos."
KM3NeT aims to finish ORCA by 2030 and ARCA by 2031. Even at roughly one-quarter deployed, the collaboration has already delivered a paradigm-shifting result: moving astronomy further into a true multimessenger era where optical observations, gravitational-wave detections, gamma rays and neutrinos together tell a fuller story of cosmic events.
KM3NeT demonstrates that to gaze at the farthest reaches of space, sometimes we must build instruments in the planet’s deepest places. As DeJong puts it: "We are literally stardust — isn’t that a fantastic concept?"
