From The Crop Circle Ship Wiki
A tetrahedron (plural: tetrahedra) is a polyhedron composed of four triangular faces, three of which meet at each vertex. A regular tetrahedron is one in which the four triangles are regular, or "equilateral," and is one of the Platonic solids.
In the Crop Circle Ship, the tetrahedron shape is used in the Large Tetrahedron Structure.
A tetrahedron can be folded from a single sheet of paper.