KM3NeT: The Mediterranean’s Undersea Neutrino Telescope and Its Landmark 220 PeV Detection
KM3NeT is an ambitious dual-detector undersea neutrino telescope located in the Mediterranean Sea, with ORCA near Toulon, France, and ARCA near Capo Passero, Italy. It employs Digital Optical Modules, each housing 31 small photomultiplier tubes within spherical units, and spans a cubic kilometer of water to detect neutrinos through the Cherenkov light they emit.
In 2023, KM3NeT recorded the most energetic neutrino observed to date, an event named KM3-230213A, with an energy of 220 PeV. This neutrino likely originated from a distant blazar, providing crucial evidence for natural particle accelerators vastly more powerful than the Large Hadron Collider.
The ORCA detector focuses on studying the neutrino mass hierarchy by analyzing the flavor oscillations of neutrinos as they traverse the Earth, while the larger ARCA instrument is designed to capture ultra-high-energy cosmic neutrinos. The project aims to advance multi-messenger astronomy by using the Mediterranean as the world’s largest high-energy physics laboratory, integrating neutrino observations with electromagnetic and gravitational-wave data.
As of early 2025, about 25% of the planned detector elements have been deployed, with full completion targeted by 2030 for ORCA and 2031 for ARCA. The endeavor faces significant engineering challenges including operating under deep-sea pressures of approximately 350 atmospheres, combating highly corrosive saltwater environments, maintaining autonomous operation for decades, and developing novel fiber-optic data transmission systems. Notably, a 2025 seafloor power-network failure at ARCA required a complex robotic repair.
The detection of the 220 PeV neutrino event highlights KM3NeT’s capability to map cosmic sources through neutrinos, reaffirming the existence of powerful natural particle accelerators and marking a significant step forward in high-energy astrophysics.
