Hyperboloids of two sheets are determined by the equation \(\frac{x^2}{a^2}+\frac{y^2}{b^2}-\frac{z^2}{c^2}=-1\) with \(a,b,c\) positive real numbers. The sections with planes normal to the \(z\)-axis are ellipses. For \(a=b\) the surface is an surface of revolation of an hyperbola.