KM3NeT: Neutrino Astronomy Under the Mediterranean

In the Ionian Sea, 80 kilometres off the Sicilian coast, a sparse lattice of vertical optical fibres hangs suspended between weights on the seafloor and buoys near the surface. Each fibre — a “detection unit” string — carries 18 digital optical modules (DOMs), glass spheres containing photomultipliers that record the Cherenkov light from charged particles traversing the water. A second such installation sits off Toulon, France, at a shallower depth. Together they form KM3NeT: the Cubic Kilometre Neutrino Telescope, Europe’s answer to IceCube.

Construction began in the mid-2010s. As of 2026, with roughly a third of the array deployed, KM3NeT is producing its first physics results — astrophysical neutrino candidates from the Galactic plane, atmospheric neutrino measurements aimed at resolving the mass ordering, and a separate class of events that open a research programme distinct from IceCube’s.

This post describes the two detectors (ARCA and ORCA), the physics goals that motivate them, and their complementarity with the established IceCube programme at the South Pole.

Two detectors, two goals

KM3NeT is not a single detector but a distributed infrastructure with two physically separate sites, each optimised for a different physics programme.

ARCA (Astroparticle Research with Cosmics in the Abyss) is located 3500 metres deep, 80 km east-southeast of Capo Passero, Sicily. When complete, it will span 230 strings arranged on a roughly cubic-kilometre volume. Its string spacing is wide (~90 m between adjacent strings), optimising sensitivity to high-energy astrophysical neutrinos in the TeV–PeV regime. ARCA is, in design, a close analogue of IceCube — an array meant to reconstruct the direction of high-energy cosmic neutrinos from extragalactic sources.

ORCA (Oscillation Research with Cosmics in the Abyss) is located 2475 metres deep, 40 km south of Toulon, France. When complete, it will span 115 strings with much tighter spacing (~20 m). The denser instrumentation gives sensitivity to GeV-scale atmospheric neutrinos — exactly the regime where matter effects on oscillation through the Earth’s mantle become observable. ORCA’s primary physics goal is the neutrino mass ordering: determining whether $m_3$ is the heaviest or the lightest mass eigenstate.

Both detectors use identical DOM technology — 31 multi-anode 3-inch photomultipliers per sphere, giving per-module direction information and noise rejection that single-PMT designs cannot match. Both share Earth-based computing and analysis infrastructure.

Water versus ice

KM3NeT’s principal technological choice is water instead of ice as the detection medium. This has tradeoffs.

Advantages of water:

• Better angular resolution for track events. Light scattering is much lower in water than in ice, so photons travel in straighter lines. For muon-track events, this gives ~2× improvement in directional reconstruction — 0.1° compared to IceCube’s ~0.3°.
• The detectors are in international waters with no permitting issues for deployment.
• Access to the strings is possible (via ship) for maintenance and repair.
• The ocean floor is more stable than ice, with no plate-motion or ice-flow concerns.

Disadvantages of water:

• String positions drift with currents; they must be constantly monitored using acoustic transceivers.
• DOMs must withstand 250+ atm pressure at depth — demanding mechanical design.
• Deployment requires expensive ship time and specialised ROVs.
• Bioluminescence from deep-sea organisms adds background to the low-level photon flux.
• ${}^{40}$K decay in seawater produces a steady background photon rate.

The net effect: KM3NeT has better angular resolution than IceCube for certain event classes, but harder deployment and maintenance. It sees the sky IceCube cannot reach (the Northern-sky view is the important complement), but its total effective area grows more slowly as strings are added.

ARCA physics

The ARCA programme is focused on high-energy astrophysical neutrinos — the same physics IceCube pioneered from 2013 onwards. But the Mediterranean location gives ARCA a crucial advantage: the Galactic centre is in its sky.

IceCube, at the South Pole, sees the Galactic centre constantly, but sees it only through the Earth — and for the TeV-energy events that IceCube sees best, the Earth is not transparent, so the Galactic centre is effectively in IceCube’s backward hemisphere. KM3NeT/ARCA, in the Northern hemisphere, sees the Galactic centre rising and setting, giving direct sensitivity through the astronomically relevant “upward-going” geometry.

The first ARCA astrophysical neutrino candidates, published in preliminary form in 2023–2024, include events consistent with the diffuse Galactic-plane emission that IceCube also reported in 2023. A combined ARCA-plus-IceCube analysis in the early 2030s should identify specific Galactic and extragalactic source populations with cleaner statistical power than either experiment alone.

ORCA physics: the mass-ordering measurement

ORCA’s primary goal is determining the neutrino mass ordering by precision measurement of atmospheric neutrino oscillations.

The physics setup is straightforward in principle. Atmospheric neutrinos — produced by cosmic-ray interactions in the upper atmosphere — arrive at the detector from all directions. Those arriving from below have traversed some portion of the Earth. The matter effects on their oscillation depend on the mass ordering: normal ordering enhances oscillation for neutrinos (suppresses for antineutrinos) in a specific angular/energy region; inverted ordering flips this pattern.

ORCA’s energy threshold is approximately 2–5 GeV — matched to the relevant oscillation regime. Its 115-string geometry, with 18 DOMs per string and compact inter-string spacing, gives good event reconstruction at those energies. The analysis looks for the characteristic zenith-angle + energy pattern that distinguishes orderings.

ORCA’s 2023 preliminary results show evidence for the normal ordering at approximately 1–1.5σ — consistent with hints from JUNO, long-baseline experiments, and global fits. Full sensitivity for 3σ determination is expected around 2028 after complete deployment and 3 years of data.

In parallel, ORCA measures the atmospheric mixing angle ${\theta }_{23}$ to better than 1° precision and provides an independent constraint on $|\Delta m_{31}^2|$.

Deployment status

Deployment has been steady but slower than initial projections. As of early 2026:

• ARCA: 33 strings deployed, approximately 14% of final target
• ORCA: 18 strings deployed, approximately 16% of final target

Both sites are currently taking physics data with partial-array configurations. The 2026–2028 deployment plan targets 60% coverage at ARCA and full coverage at ORCA, which together would enable definitive mass-ordering measurement and substantially sharpen the astrophysical programme.

Deployment is constrained primarily by ship availability. Each KM3NeT string requires several days at sea for deployment, using ROV (remotely operated vehicle) positioning and anchor placement. Two to four strings are deployed per ship campaign. A full ARCA requires approximately 230 strings, implying multi-year deployment schedule even with dedicated ships.

Complementarity with IceCube

The two global neutrino telescopes are complementary, not competing.

Sky coverage: IceCube sees mostly the Southern sky (Galactic plane below Earth). KM3NeT sees mostly the Northern sky (Galactic plane above). Together, they give 4π sky coverage.

Angular resolution: KM3NeT/ARCA’s superior angular resolution for track events lets it point back to astrophysical sources with better precision. IceCube’s higher statistics let it accumulate larger samples at lower resolution.

Energy range: Both cover TeV–PeV. IceCube has larger target mass; KM3NeT/ARCA has better angular reconstruction.

Mass ordering: ORCA is dedicated to this measurement via atmospheric neutrinos; IceCube-DeepCore can also contribute but with coarser resolution. The two approaches cross-check each other.

Alerts: When one telescope observes a suggestive event, the other can provide independent follow-up and combined sensitivity. The 2023 Galactic plane detection was strengthened by ARCA’s parallel observations.

The broader landscape

KM3NeT, IceCube, and (in the near future) the proposed P-ONE detector off Canada’s West Coast together form a global neutrino astronomy infrastructure. Each site has its particular strengths; combined analyses across all three produce more sensitive measurements than any single detector.

For specific physics goals, this international cooperation is already paying dividends:

• Sky coverage for point-source searches benefits from the full-sky complement
• Transient events (gamma-ray bursts, tidal disruption events) can be followed up across all detectors, and the joint analysis eliminates sky-coverage biases
• Systematic cross-checks using different detector media reduce risk of a single-detector systematic masquerading as physics

By the 2030s, neutrino astronomy will have matured from a single-observatory discipline (IceCube alone) to a multi-observatory discipline analogous to optical astronomy in the era of Hubble-plus-JWST-plus-ground-based.

KM3NeT is Europe’s contribution to that infrastructure. Its deployment over the coming decade, combined with complementary instruments elsewhere, will put the first complete all-sky high-energy neutrino observatory into operation.