KM3NeT: how Europe is turning the Mediterranean into a neutrino telescope

Off the coasts of Sicily and southern France, two arrays of glass spheres are being slowly sunk into the dark blue water of the Mediterranean. Each glass sphere holds 31 photomultiplier tubes pointed in different directions. The spheres are threaded onto vertical cables, anchored to the seabed, and held upright by buoyancy. When the construction is complete, the two arrays will contain thousands of these spheres distributed across kilometers of seafloor — and together they will form KM3NeT, the Cubic Kilometre Neutrino Telescope.

KM3NeT is Europe’s answer to IceCube. Where IceCube freezes its detectors into the South Polar ice, KM3NeT sinks them into the deep Mediterranean. Where IceCube views primarily the southern sky, KM3NeT views the northern sky. And where IceCube is a single uniform array, KM3NeT is being built as two complementary detectors: ARCA, optimized for high-energy astrophysical neutrinos, and ORCA, optimized for lower-energy atmospheric ones.

Two detectors, two physics goals

The KM3NeT collaboration decided early to split the project into two distinct detectors at two different sites, each tuned for different physics.

ARCA — Astroparticle Research with Cosmics in the Abyss — sits about 100 kilometers offshore from Portopalo di Capo Passero, Sicily, at a depth of about 3.5 kilometers. It is the larger of the two, eventually planned to span a cubic kilometer or more, and is designed for the highest-energy neutrinos: TeV, PeV, and even higher. ARCA’s mission is neutrino astronomy — identifying point sources, measuring the diffuse extragalactic flux, hunting transient events.

ORCA — Oscillation Research with Cosmics in the Abyss — sits about 40 kilometers offshore from Toulon, France, at about 2.5 kilometers depth. It is more densely instrumented than ARCA over a smaller volume, tuned for atmospheric neutrinos in the GeV energy range. ORCA’s mission is oscillation physics — particularly the neutrino mass ordering, which produces a measurable signature in the way atmospheric neutrinos oscillate as they cross the Earth.

Both detectors share the same hardware design — the same glass spheres, the same anchoring scheme, the same data acquisition system. Only the spacing differs. The collaboration that builds them is the same.

Why deep seawater

Building a kilometer-scale neutrino telescope requires an enormous, transparent volume of natural detector medium. There are three viable choices. Antarctic ice (used by IceCube) is exceptionally clear but optically scatters light, blurring directional information slightly. Lake water (used by Baikal-GVD in Russia) is darker and less scattering than ice but has less stable optical properties. Deep seawater offers a third option: dark, scattering-free, but with more bioluminescence and more radioactive background from potassium-40 dissolved in saltwater.

The KM3NeT team chose seawater for several reasons. Sea-level access makes deployment and maintenance easier than the South Pole. The Mediterranean is calm, deep, and close to major European research institutions. And the better angular resolution of water more than compensates for the additional backgrounds.

The downside is that the optical modules sit in active seawater. Bioluminescent organisms — small jellyfish, krill, and microorganisms — drift past the detectors, flashing as they go. Marine sediment occasionally settles on the optical surfaces. Strong currents can shake the detector strings. None of these is a fatal problem, but each adds engineering complexity that IceCube does not have to deal with.

The 220 PeV event

In February 2025, the KM3NeT collaboration published a paper in Nature reporting an extraordinary event detected by ARCA on February 13, 2023. A single, extremely high-energy muon had crossed the partially-deployed detector at almost horizontal angle, depositing a vast track of Cherenkov light. The reconstructed muon energy was approximately 120 PeV, and the inferred parent neutrino energy was approximately 220 PeV — about thirty times higher than any neutrino energy previously measured.

If the interpretation holds up under scrutiny, this would be the highest-energy neutrino ever detected, by a significant margin.

The result has generated active discussion. The reconstructed track is unambiguously real and the energy estimate is robust, but the source of the parent neutrino is not yet identified — the angular resolution is not sharp enough to pinpoint a specific astrophysical object. Several follow-up campaigns are underway to look for any flaring extragalactic source that might be consistent with the event’s direction and timing.

Whether this single event represents a fluctuation, a hint at a high-energy population that IceCube has been narrowly missing, or something more exotic, will become clearer as KM3NeT continues to grow and accumulate exposure.

How the optical modules work

Each KM3NeT detection unit is a glass-sphere digital optical module (DOM) — a 17-inch pressure-resistant sphere containing 31 small photomultiplier tubes of 3-inch diameter, arranged so that the angular acceptance is broad. The use of many small photomultipliers in each module is a significant departure from IceCube, which uses one large photomultiplier per module.

The advantage of the multi-PMT design is directional sensitivity from each individual module. When light arrives at a DOM, the relative timing and intensity across the 31 PMTs constrains the arrival direction much better than a single large PMT can. Combined with the spatial array of many DOMs, this produces angular resolutions of a few tenths of a degree for track events — significantly better than what IceCube achieves.

Each DOM is threaded onto a vertical mechanical cable along with thirty-three others, forming a detection unit about 700 meters tall. The detection units are anchored on the seabed, held upright by an underwater float at the top, and connected by submarine cables to a shore station. ARCA is planned to have 230 detection units, ORCA 115. Construction is ongoing — both detectors are partially deployed and already taking physics-quality data.

What KM3NeT brings to the field

KM3NeT’s value to neutrino physics is threefold.

Different sky coverage from IceCube. Because the Mediterranean is in the northern hemisphere, KM3NeT is best at observing sources in the southern celestial sky — exactly opposite IceCube’s coverage. The two detectors are complementary observatories. The galactic center, in particular, is well-placed for KM3NeT.

Better angular resolution. The combination of seawater and the multi-PMT DOM design gives KM3NeT an angular resolution advantage of a factor of a few at most energies. For point-source identification, this matters considerably.

Mass ordering through ORCA. ORCA’s atmospheric neutrino program offers a different, independent path to the mass ordering than JUNO or DUNE. Cross-checks between independent methods will be important if any of the experiments produces marginal results.

The construction timeline

ARCA’s construction has been ongoing since 2015, with the first detection unit deployed in 2015 and additional units added periodically. As of 2026, about a third of the planned ARCA volume is operational and taking data. The full deployment is targeted for the late 2020s.

ORCA construction has run on a similar timeline, with first instrumentation deployed in 2017 and additional units following. ORCA is somewhat farther along — about half-completed as of 2026 — and is already producing competitive oscillation results from atmospheric neutrinos.

A complementary observatory

KM3NeT is being built in parallel with several other major neutrino projects: IceCube-Gen2, the planned expansion at the South Pole; Baikal-GVD, a similar kilometer-scale telescope in Lake Baikal; and P-ONE, a proposed deep-Pacific observatory off the coast of Canada. Together these projects will form a global network of high-energy neutrino observatories, all viewing different patches of sky and providing cross-checks against one another.

The era of single-site neutrino astronomy is ending. The era of distributed, multi-detector neutrino observatories — capable of catching transient events from all directions in real time and following up with the full multi-messenger arsenal — is just beginning. KM3NeT is one of the cornerstone projects of that transition.

In the next decade, KM3NeT’s results will reshape both galactic and extragalactic neutrino astronomy. The 220 PeV event, if confirmed and joined by more like it, may already be hinting at what is to come.

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For the South Pole counterpart, see Inside IceCube and The Milky Way’s neutrino glow. For the broader astrophysics context, see Cosmic messengers from blazars, TXS 0506+056 and NGC 1068. For the mass-ordering work ORCA contributes to, see Normal or inverted.

Neutrino Times Editorial Team. (2025, October 11). KM3NeT: how Europe is turning the Mediterranean into a neutrino telescope. Neutrino Times. https://neutrino-times.com/articles/km3net-mediterranean-neutrino-telescope/

Neutrino Times Editorial Team. "KM3NeT: how Europe is turning the Mediterranean into a neutrino telescope." Neutrino Times, October 11, 2025. https://neutrino-times.com/articles/km3net-mediterranean-neutrino-telescope/.

Neutrino Times Editorial Team. "KM3NeT: how Europe is turning the Mediterranean into a neutrino telescope." Neutrino Times, 11 Oct. 2025, https://neutrino-times.com/articles/km3net-mediterranean-neutrino-telescope/.

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