The number is 6,320,430 digits long -- more than 2 million digits larger than the previous largest known prime and requires 1,500 pages to print it. 

The number turned up on a Michigan State University graduate student's computer.  Michael Shafer, 26, participated as a volunteer in an eight-year-old, worldwide project called...

...which harnessed the power of 211,000 computers, in effect creating a supercomputer capable of performing 9 trillion calculations per second. 

Mersenne primes are a special category, expressed as 2^p  - 1 where p is also a prime number. Shafer's discovery can be written as...

2^20,996,011 - 1