While the concept of the "12th man" may not have originated in Seattle, there isn't a group of fans in the NFL who have more of a direct impact on the game than those who cheer for the Seahawks.
The noise reaches insane levels, forcing opposing offenses into committing false start penalties and a sense of general frustration, something Green Bay Packers QB Aaron Rodgers can attest to (h/t ESPN):
Those fans are really intelligent fans. They get so stinking loud out there. They do a really good job of giving the defense that advantage when we have to go on some silent counts or when we're trying to hear each other. They should be commended for that.
We practice with sound at practice, and you try to simulate some of that. But I think you know when you play at a stadium like Seattle -- Seattle and Kansas City are probably the two loudest outdoor stadiums -- but it's definitely an extra factor and aid for that defense.
Rodgers wasn't the only NFL player to be taken with the incredible noise that Seattle fans create.
Deion Branch, who spent time with both the Seahawks and New England Patriots over his 11-year career, didn't hesitate when the Boston Globe's Zuri Berry asked him about playing in Seattle earlier this season:
"It's by far the loudest stadium [in the NFL], and it can get louder if things go bad for us on the offensive side of the ball."
For anyone who has ever been to CenturyLink Field, this should come as no surprise.