Current knowledge of the Universe is based on information carried by electromagnetic radiation, gravitational waves, neutrinos, and cosmic rays (CRs), taking advantage of a multi-messenger approach. Among those cosmic messengers, neutrinos have unique features: high-energy astrophysical neutrinos, originated via collisions of CRs with high-density matter and/or radiation fields, can cross the Universe preserving the directional information about their production site, allowing us to unambiguously identify the most efficient sources of particle acceleration, shedding light on the extreme part of the Universe.
During this seminar, in addition to describing the principles on which the currently operating instruments performing high-energy neutrino astronomy are based, I will provide an overview of the state-of-the-art of this field, highlighting the important milestones reached by the community over the last decade. However, despite the exciting times, high-energy neutrino astronomy is only at the dawn. In the coming years, afundamental role in the rapidly evolving multi-messenger field will be held by KM3NeT, a multi-purpose and multi-site neutrino observatory currently under construction at the bottom of the Mediterranean Sea. In this regard, I will describe its current status andperspectives of the forthcoming full configuration, with a focus on the KM3NeT online multimessenger activies, that may be crucial for significant breakthroughs in the future.