The word tetrahedron is made up of two Greek words meaning four seats or bases, and a tetrahedron is a solid figure bounded by four equilateral triangles. To conjure up a picture of what this means we may say that a tetrahedron is a pyramid with three sides and a base, all of which are equal.
It has been said.... The world is now the shape of a globe, the shape which gives the biggest possible bulk for its surface, but the inside of the earth is still cooling and condensing, and the internal changes are slowly changing its shape. The surface, already condensed to its utmost, will not change with the core; it cannot reduce its area, but it adapts itself to the shrinking interior by taking a shape which occupies less bulk. So the earth is to become a tetrahedron, a sort of pyramid, the shape which gives the smallest bulk for its surface. While the sphere is the figure occupying the greatest space, for a given surface, the tetrahedron occupies the least space for an equal surface, and therefore, mathematically, if the process described goes on, the earth should one day become a tetrahedron.