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.