  Weakly interacting neutrinos are ideal astronomical messengers because they
travel through space without deflection by magnetic fields and, essentially,
without absorption. Their weak interaction also makes them notoriously
difficult to detect, with observation of high-energy neutrinos from distant
sources requiring kilometer-scale detectors. The IceCube project transformed a
cubic kilometer of natural Antarctic ice at the geographic South Pole into a
Cherenkov detector. It discovered a flux of cosmic neutrinos in the energy
range from 10 TeV to 10 PeV, predominantly extragalactic in origin. Their
corresponding energy density is close to that of high-energy photons detected
by gamma-ray satellites and ultra-high-energy cosmic rays observed with large
surface detectors. Neutrinos are therefore ubiquitous in the nonthermal
universe, suggesting a more significant role of protons (nuclei) relative to
electrons than previously anticipated. Thus, anticipating an essential role for
multimessenger astronomy, IceCube is planning significant upgrades of the
present instrument as well as a next-generation detector. Similar detectors are
under construction in the Mediterranean Sea and Lake Baikal.
