  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.
