It's a Mersenne prime. I was curious so looked at the list of numbers that have been the largest known prime. It turns out that the last time the largest known

It's a Mersenne prime. I was curious so looked at the list of numbers that have been the largest known prime. It turns out that the last time the largest known prime was not a Mersenne prime was in 1989. That was 391581 x 2^216193 - 1 and had 65087 digits. There have been 19 largest known primes since then and all are Mersenne primes. Out of the 20 largest known primes, 12 are Mersenne primes. The ones that aren't are at positions 8, 9, 12, 14, 16, 18, 19, and 20.