  Neutrino telescopes can observe neutrino interactions starting at GeV
energies by sampling a small fraction of the Cherenkov radiation produced by
charged secondary particles. These experiments instrument volumes massive
enough to collect substantial samples of neutrinos up to the TeV scale as well
as small samples at the PeV scale. This unique ability of neutrino telescopes
has been exploited to study the properties of neutrino interactions across
energies that cannot be accessed with man-made beams. Here we present the
methods and results obtained by IceCube, the most mature neutrino telescope in
operation, and offer a glimpse of what the future holds in this field.
