A system administrator - not a mathematician - used a grid of computers supplied by volunteers at the University of California, Los Angeles, to find the world's largest known prime number. The immense number is made up of nearly 13 million digits.

The discovery is part of the Great Internet Mersenne Prime Search (GIMPS), a 12-year-old project that uses the computers of volunteers to find larger and larger prime numbers. The volunteer project has been focused on finding the first prime number that has more than 10 million digits.

As a prize, the Electronic Frontier Foundation is handing out $100,000 with half going to the winner and half going to charity.

A prime number is a whole number that can only be divided by one and itself. Mersenne prime numbers are a class of prime numbers named after Marin Mersenne, a 17th century French monk who studied the rare numbers 300 years ago. And Edson Smith, the system administrator at UCLA who found the largest Mersenne prime, explained that primes and even Mersenne primes are easy to find in the lower numbers, like 3 and 5, but become much more difficult to find when the numbers become long and intricate.

The prime that Smith and his team at UCLA found was 12,978,189 digits long. It's such a large number that if you printed it out, it would run 30 miles long, according to Smith, who said he believes that if you tried to read it out loud, you couldn't finish it during your lifetime.

"It's really cool for everybody involved," Smith told Computerworld US. This is an excellent demonstration of the power of the grid."

Smith explained that the GIMPS project leaders hand out potential prime numbers to teams of volunteers, such as that at UCLA, whose computers ran software designed to test the number.

The UCLA team used 75 Dell desktop PCs running Windows XP. Smith noted that if they had only had one computer running the program, the job would likely take longer than his lifetime.

"There are so few of these large prime numbers," said smith. "They're very rare and can only be discovered through computing power. It's really about the power of the grid. In a certain sense, I'm a lottery winner. There are thousands [of people] looking with tens of thousands of computers and it just happened to be us."

This isn't the first prime number to be discovered at UCLA - it's the eighth, according to the university. In 1952, UCLA Professor Raphael Robinson discovered five different Mersenne primes - reportedly the first ones to be found using computers.

GIMPS founder George Woltman said the organisation will now offer a $150,000 award for the first person or group to find the first 100-million-digit prime number.