KM3-230213A: The Highest-Energy Cosmic Neutrino

High-energy neutrinos provide a unique probe of the most extreme nonthermal processes in the Universe. The detection of the ultra-high-energy neutrino event KM3-230213A by the KM3NeT neutrino telescope, with an inferred parent-neutrino energy of O(1017 eV), represents a major step forward in neutrino astronomy. In this review, we summarize the main properties of the event and discuss its interpretation within different astrophysical and cosmological scenarios, including active galactic nuclei, transient sources, and cosmogenic neutrinos, highlighting the role of multimessenger constraints. Despite extensive follow-up observations, no unambiguous electromagnetic counterpart has been identified, and the event remains in tension with the absence of comparable detections in other experiments. This situation leaves open the possibility of more complex source scenarios or contributions from physics beyond the Standard Model. We also discuss the implications for the ultra-high-energy neutrino landscape and the prospects for future observations with next-generation detectors. KM3-230213A provides a unique opportunity to probe particle acceleration and neutrino production at energies far beyond the reach of terrestrial experiments.