The Great Internet Mersenne Prime Search (GIMPS), operating since 1996, has turned up five extremely large prime numbers so far. The fifth and largest was discovered in November 2001 by 20-year-old Michael Cameron of Owen Sound, Ontario. Mersenne primes can be expressed as 2P - 1, where P is itself a prime number. Cameron's is 213,466,917 - 1, which would take four million digits to write out. His computer spent 45 days discovering that his number is a prime; altogether the GIMPS network expended 13,000 years of computer time eliminating other numbers that could have been the 39th Mersenne.
The 38th Mersenne prime, a mere two million digits long, earned its discoverer (Nayan Hajratwala of Plymouth, Mich.) a $50,000 reward for being the first prime with more than a million digits. A prime with 10 million digits will win someone $100,000.