Who Are Golf's "Invisible Man" Guys?

A bizarre scene is unfolding at today's third round action at the AT&T National. After violent storms uprooted several trees (including a 75 foot tree falling across the fairway on 14) and delayed play, the course has been overrun by thousands of "Invisible Man" guys.

As you can see from this shot of today's play, virtually the entire gallery at Congressional Country Club in Bethesda, Maryland has gotten in on the act. If you listen to the broadcast, the fans have really dedicated themselves to the role as everyone has gone quiet, even the "Get in the hole!" guys. Who are these guys and what are they playing at? [ PGA.com]