M38 is the short hand used to refer to the 38th Mersenne prime. Specifically it is 26,972,593-1. This number was discovered to be prime on 1999-06-01 by Nayan Hajratwala, using Prime95 written by George Woltman. The number is 2,098,960 decimal digits long.
The prime was independently verified by David Willmore using Mlucas program written by Ernst Mayer using two weeks of computer time donated by Aerial Communications on a 500 MHz DEC Alpha workstation.
This prime number was the fourth record prime found by the GIMPS project.