Pi (π) is the ratio of the circumference of a circle to its diameter. Pi is an irrational number, which means that it is a real number that cannot be expressed by a simple fraction. That's because pi is what mathematicians call an "infinite decimal" — after the decimal point, the digits go on forever and ever.

This tool lets you search through the first 10 million digits of Pi [Source: Princeton]. For a more robust search through 2 Billion primes, see Subidiom's Digits of Pi tool.