A new prime number has been discovered — and it's the biggest one known by almost five million digits.

A prime number is a whole number greater than one that can only be divided evenly by itself and one. The best known primes are the small ones such as 2, 3, 5, and 7. New, very large ones are difficult to find and of great interest to mathematicians.

The newest prime number, also known as M74207281, has 22,338,618 digits and is the number you get when you multiply 74,207,281 twos together and then subtract one, says the Great Internet Mersenne Prime Search, which announced the discovery.

You can download a zip file of the entire number here.

That makes it a special kind of number called a Mersenne prime. All Mersenne primes can be produced by multiplying together a certain number of twos and subtracting one. Only 49 Mersenne primes have ever been discovered, including the new one.

Anyone who wants to participate in the Great Internet Mersenne Prime Search can download software called Prime95 that allows the project to use their work or home PC. (Andrey Popov/Shutterstock)

The latest discovery was made by the Great Internet Mersenne Prime Search, a project that harnesses the power of hundreds of thousands of ordinary computers, volunteered by their owners, over the internet.

In this case, the computer that found the number was owned by the University of Central Missouri and volunteered by Curtis Cooper, a professor of computer science. It took the PC 31 days of non-stop computing to prove that the number was prime. That happened back in September, "but it remained unnoticed until routine maintenance data mined it," GIMPS reported.

Once it was noticed, the discovery was verified independently by other people with different software and hardware, and announced on Jan. 7, when GIMPS celebrated its 20th anniversary.

GIMPS bills itself as the largest continuously running "grassroots supercomputing" project on the internet. The project has discovered all 15 of the largest known Mersenne primes, including one found by a volunteer in Canada in 2001.

Anyone who wants to participate can download software called Prime95 that allows the project to use their work or home PC. In return, if your computer makes a new discovery, you can win a cash reward of up to $50,000.