Square the Circle
To attempt an impossibility. The allusion is to the mathematical question whether a circle can be made which contains precisely the same area as a square. The difficulty is to find the precise ratio between the diameter and the circumference. Popularly it is 3.14159 ... the next decimals would be 26537, but the numbers would go on ad infinitum.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894