The Fullerenes, discovered in 1985 by Robert Curl, Harold Kroto and Richard Smalley at the University of Sussex and Rice University, are a family of carbon allotropes named after Richard Buckminster Fuller and are sometimes called buckyballs, when in a spherical configuration. They are molecules composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Cylindrical fullerenes are called carbon nanotubes or buckytubes. Fullerenes are similar in structure to graphite, which is composed of a sheet of linked hexagonal rings, but they contain also pentagonal (or sometimes heptagonal) rings that prevent the sheet from being planar.