  The discovery of the peculiar supernova (SN) 1998bw and its possible
association with the gamma-ray burst (GRB) 980425$^{1,2,3}$ provide new clues
to the understanding of the explosion mechanism of very massive stars and to
the origin of some classes of gamma-ray bursts. Its spectra indicate that
SN~1998bw is a type Ic supernova$^{3,4}$, but its peak luminosity is unusually
high compared with typical type Ic supernovae$^3$. Here we report our findings
that the optical spectra and the light curve of SN 1998bw can be well
reproduced by an extremely energetic explosion of a massive carbon+oxygen (C+O)
star. The kinetic energy is as large as $\sim 2-5 \times 10^{52}$ ergs, more
than ten times the previously known energy of supernovae. For this reason, the
explosion may be called a `hypernova'. Such a C+O star is the stripped core of
a very massive star that has lost its H and He envelopes. The extremely large
energy, suggesting the existence of a new mechanism of massive star explosion,
can cause a relativistic shock that may be linked to the gamma-ray burst.
