Astroparticle Physics with High Energy Neutrino Telescopes

The first optical modules of the Baikal high energy neutrino telescope have recently been deployed. Commissioning of the AMANDA, DUMAND and NESTOR detectors will follow soon. Before discussing the detectors we review the arguments that pinpoint $0.1\rm~km^2$ as the natural scale of a neutrino telescope. Though present detectors do not quite reach this goal, their techniques, if successful, can be exploited to build km$^2$ detectors for a cost not exceeding one hundred million dollars. Motivations for the construction of km$^2$ deep underground detectors include i) neutrino astronomy and the search for cosmic accelerators, ii) neutrino oscillations using the atmospheric neutrino beam, iii) the search for neutrinos from the annihilation of dark matter particles in our galaxy, iv) the possibility to observe thermal neutrino emission from supernovae, and v) to make the serendipitous discovery. No astronomical telescope, detecting photons of any wavelength, has ever viewed sites in the Universe shielded by more than a few hundred grams of matter.