  A sudden steepening of the cosmic-ray energy spectrum (the knee) is observed
at an energy of about 3 PeV (1 PeV = $10^{15}$ eV). The experimental study of
the PeV cosmic rays has intensified greatly during the last 3 years. The recent
results on extensive air showers allow us to conclude that: a) the knee has an
astrophysical origin; b) the `sharpness' and the fine structure of the knee
rule out `Galactic Modulation' as the origin of the knee; c) most likely the
knee is the result of the explosion of a single, recent, nearby supernova.
