Dustin quoted Here's looking at Euclid by Alex Bellos
There is no practical reason to know pi to 72 digits, or to 35 digits for that matter. Four decimal places are enough for the engineers of precision instruments. Ten decimal places are sufficient to calculate the circumference of the Earth to within a fraction of a centimeter. With 39 decimal places, it is possible to compute the circumference of a circle surrounding the known universe to within an accuracy of a radius of a hydrogen atom.