The Universe Seen by a Telescope 1 Kilometer Deep in the Ocean - Neutrino Astronomy: A New Window Opened by KM3NeT

From the Depths of the Sea to the Beginning of the Universe

The European deep-sea neutrino telescope "KM3NeT" is receiving "raw messages" from extreme phenomena occurring at the farthest reaches of the universe. Strings of glass spheres with embedded sensors form "glowing threads" that rise in a grid pattern in the darkness of the Mediterranean Sea. KM3NeT aims to capture the flashes of Cherenkov light emitted by neutrinos in water with this underwater 3D telescope, bringing us closer to the origins of the universe. A feature reported by Phys.org on October 24, 2025, vividly summarizes its goals and latest achievements. Phys.org

Record-breaking "Ghost Particles"

In the early hours of February 13, 2023, KM3NeT captured the highest-energy neutrino candidate ever observed. The event is named "KM3-230213A." Its energy is approximately 220 PeV, about 30 times the previous record. A paper published in Nature details the estimated energy of the detected muon (around 120 PeV, with a large margin of error) and the characteristics of the event. Although the celestial body from which it originated has not yet been identified, potential candidates include jets from blazars (active galactic nuclei) and "cosmogenic neutrinos" produced by interactions between cosmic rays and background photons. Nature

What Kind of Telescope Is It? — ARCA and ORCA

KM3NeT is composed of two "seabed campuses."

• ARCA (off Sicily): Specializes in searching for high-energy neutrinos from deep space.

• ORCA (off Toulon): Focuses on precise measurements of neutrino oscillations and mass ordering.
  Each has numerous lines extending vertically from the seabed, equipped with basketball-sized optical modules to detect faint blue light in the ocean. Over a thousand sensors are already operational, with plans to expand to several thousand in the coming years. The concept of using the ocean as a vast transparent medium for detection aligns with the approaches of IceCube in Antarctica and Super-Kamiokande in Japan. Phys.org

Why Chase Neutrinos?

Neutrinos, which have no charge and almost no mass, pass through stars and planets, carrying nearly "raw data" from the universe. This makes them key to understanding extreme environments such as those around black holes, supernova explosions, and cosmic ray acceleration sites. Furthermore, they provide crucial hints to one of the universe's greatest mysteries: the asymmetry between matter and antimatter, through questions like the ordering of neutrino masses (normal or inverted) and whether neutrinos are their own antiparticles (Majorana particles). Phys.org

"Coincidence" or a Sign of "New Physics"?

KM3-230213A, with its unexpectedly high energy, was so shocking that researchers had to "redo" their analysis. To narrow down the origin candidates, improving directional accuracy and simultaneous detection with other wavelengths and messengers are essential. General media like Wired are also focusing on whether this event is a "random deviation" or the "tip of the iceberg." The answer lies in the next event—reproducibility and statistics. WIRED

The "European Advantage" of Underwater Infrastructure

KM3NeT is a flagship European research infrastructure supported by the EU and various countries. The ability to deploy equipment in the harsh environment of the deep sea while continuing to expand provides the stability essential for long-term surveys. Phys.org's feature also emphasizes the technical boldness and scale of international cooperation. Phys.org

How Social Media Sees It: The "Double Helix" of Enthusiasm and Skepticism

• Official Announcements' Excitement: KM3NeT's official X shares ongoing research considering cosmogenic origins and the status of the next phase of analysis. The community's expectations for a new era of "listening to the universe from the deep sea" have grown. X (formerly Twitter)

• Academia's Caution: University and research institution accounts maintain a cautious stance, noting that "statistical randomness cannot yet be ruled out." There were also views warning against excessive generalization. X (formerly Twitter)

• Reddit's Buzz: On r/space and r/physicsmemes, the extraordinary figure of 220 PeV sparked humorous astonishment, with comments like "Lorentz factor is astronomical." Meanwhile, concerns about unidentified origins and systematic errors were also discussed. Reddit

• Reactions in the Japanese Sphere: Domestic media and science-related accounts also spread the keywords "30 times the previous record" and "underwater telescope." The use of relatable metaphors (like "energy of a ping-pong ball") helped it reach a general audience. X (formerly Twitter)

The Not-So-Small "Seed of Controversy"

Amid the excitement, IceCube's long-term analysis suggests that "the flux at the 100 PeV level is much smaller than expected," emphasizing the rarity of KM3-230213A. If KM3NeT's interpretation is correct, the discussion of whether "cosmic ray composition is not primarily protons" could reignite. The ongoing task is to carefully compare differences in sensitivity, analysis methods, and handling of systematics. A healthy tension serves as a driving force to strengthen the results. Press Release Distribution and News Release Distribution Service No.1｜PR TIMES

The Next Move: Multi-Messenger and Mass Production

KM3NeT will continue to add modules, enhancing directional resolution and sensitivity. Collaboration with IceCube, Baikal-GVD, gamma-ray and X-ray telescopes, and gravitational waves could allow individual neutrinos to be reconstructed three-dimensionally like a "case file." If a blazar origin becomes significant, we can directly delve into the sites of cosmic ray acceleration, and if cosmogenic origins are confirmed, we can explore the composition, distribution, and interaction history of cosmic rays with the cosmic background light. A single "point" will eventually become a "map." Phys.org

Conclusion: The Long Message from Space Carried by the Blue Light of the Deep Sea

From the unexpected "observation site" of the seabed, the most powerful class of neutrinos in the universe is becoming visible. The most important tasks at present are (1) additional events for reproduction, (2) multi-wavelength and multi-messenger simultaneous observations to help identify origins, and (3) cross-checks with other experiments. Both enthusiasm and skepticism are necessary. When these align, KM3-230213A will truly become the "first message from space." Phys.org