  Our numerical calculations on small carbon clusters point to the existence of
a metastable three-dimensional eight-atom cluster C$_8$ which has a shape of a
six-atom triangular prism with two excess atoms above and below its bases. We
gave this cluster the name "prismane". The binding energy of the prismane
equals to 5.1 eV/atom, i.e., is 0.45 eV/atom lower than the binding energy of
the stable one-dimensional eight-atom cluster and 2.3 eV/atom lower than the
binding energy of the bulk graphite or diamond. Molecular dynamics simulations
give evidence for a rather high stability of the prismane, the activation
energy for a prismane decay being about 0.8 eV. The prismane lifetime increases
rapidly as the temperature decreases indicating a possibility of experimental
observation of this cluster.
