The Navy did it.
According to the Associated Press, the mine found floating in Puget Sound between Brownsville and Bainbridge Island on Thursday was leftover from a 2005 Navy excercise near Keyport.
The Navy blew up the mine shortly after it was discovered, but because there was not a secondary explosion, they were able to determine that it was inert. Fortunately for those of us who couldn't be there, KIRO took video....
The Navy doesn't know if this is the only stray mine floating around, so they're going to keep looking. Good luck with that.