Title: High-Energy Cosmic Neutrinos - Elusive Particles to Unravel Elusive Mysteries

Abstract: New frontiers of astroparticle physics have been opened by IceCube's discovery of high-energy cosmic neutrinos. Their origin is a new mystery in the field, and solving this problem may enable us not only to understand the physics of astrophysical sources but also to obtain important clues about the old mystery, the origin of cosmic rays, and to utilize neutrinos as probes of neutrino properties, dark matter, and fundamental physics. In this colloquium, I summarize the latest results of IceCube observations and discuss their implications with emphases on multi-messenger approaches. The topics will include the story about IceCube-170922A that coincided with a blazar flare and a grand-unified cosmic particle model we recently proposed.