It's been dubbed M77232917 – and if you're wondering why it needs a codename, well, we'd be here all day typing out the 23 million digits that make it up.
If it's been a while since high school math class, here's a quick refresher: Prime numbers are those that are only divisible by 1 and themselves. Small primes are fairly easy to identify through trial and error – 6 can be divided by 2 and 3, so it isn't prime, but 7 is – but as you look at larger and larger numbers it gets less obvious. It might take you a while to figure out, for example, that 11,319,033 isn't prime because it can be divided by 213 and 53,141. Finding the really large primes is a task best left up to computers running software like GIMPS.
As the MP in its acronym suggests, GIMPS is specifically searching for a rare class of prime numbers called Mersenne Primes. These are numbers that are one less than a power of 2, expressed as Mn = 2n - 1. That means that the newcomer M77232917 is calculated through a chain of 77,232,917 twos, and then subtracting 1. It's only the 50th known Mersenne Prime ever identified, and it's made up of 23,249,425 digits.