Messengers of the infinitely small, neutrinos provide us with valuable insights into the fundamental laws of physics. Messengers of the infinitely large, traveling on cosmological distances, they are privileged probes of cataclysmic astrophysical phenomena. Neutrino Telescopes, buried in the Mediterranean abyss (ANTARES) or the ice of the South Pole (IceCube), are trying to meet this double challenge. These detectors consist of a 3D matrix of photomultipliers that detect the Cherenkov light emitted by the charged particles produced when neutrinos interact inside or around the detector. After a brief historical introduction, I will review the latest constraints obtained by the first generation deep-sea neutrino telescope ANTARES and the expectations from the next generation detector KM3NeT, both immersed in the Mediterranean Sea. In this context, synergies with Earth and Sea sciences will be mentioned. I will then discuss the potential of neutrino telescopes for the determination of the neutrino mass ordering through oscillation studies of atmospheric neutrinos in the GeV range (KM3NeT/ORCA in the Mediterranean Sea and IceCube/PINGU to be buried in the Ice at the South Pole).