Claim: The monkey saddle, belonging to the class of saddle surfaces, has an isolated umbilical point with zero Gaussian curvature at the cylindrical coordinates.
Evidence: It belongs to the class of saddle surfaces, and its name derives from the observation that a saddle for a monkey would require two depressions for the legs and one for the tail. The monkey saddle has an isolated umbilical point with zero Gaussian curvature at the origin, while the curvature is strictly negative at all other points. The origin of the system is the point where all three coordinates can be given as zero.