Io orbits within a belt of intense radiation known as the Io plasma torus. Hyperbolic elements of the modular group act as Anosov diffeomorphisms of the torus. This produces a geometric object called the Clifford torus, a surface in 4-space. Some paleoanthropologists distinguish between frontal torus and supraorbital ridge. A [[toroidal graph]] is a graph that can be embedded without crossings on the [[torus]]. Io's volcanic ejecta also produce a large plasma torus around Jupiter. In another direction, toric varieties are algebraic varieties acted on by a torus. In theory, a nanotorus is a carbon nanotube bent into a torus (doughnut shape). Examples are spaces like the sphere, the torus and the Klein bottle. Overall it more resembles a torus or doughnut than a belt. It is diffeomorphic to a regular torus but not isometric. The Princeton Large Torus, a tokamak, operated from 1975. An ideal coil is distributed evenly all around the circumference of the torus. The Joint European Torus and DIII-D tokamaks' vacuum vessels are made of Inconel. It is less common than bony growths occurring on the palate, known as torus palatinus.