Mexican-hat (Ricker) surface
Equation: z = (1 - (x^2 + y^2)/8) * exp(-(x^2 + y^2)/8)
The Mexican hat is the Laplacian of a Gaussian up to a constant. A positive center is ringed by a negative trough, then fades to 0.
It is used in computer vision (blob detection) and seismology (the Ricker wavelet).