In the violent game we enjoy, played by men of courage, it's the sound you never want to hear and the sight you never want to see.
The "smack" of football helmets colliding at high velocity, a young man collapsing to the grass and lying motionless as medical personnel sprint onto the field.
Stone silence in the crowded stadium, aid being administered for five minutes, 10 minutes and longer.
And members of both teams, the Raiders and Steelers, kneeling in prayer.
No flag, but plenty of fear.
No penalty was called Sunday afternoon when Pittsburgh safety Ryan Mundy lunged and slammed the crown of his helmet into the face mask of Raiders receiver Darrius Heyward-Bey, twisting Heyward-Bey's neck and rendering him unconscious in the end zone.
This is the kind of evidence that should persuade the $9 billion industry that is the NFL to seek and train and monitor -- and pay for -- the best possible physicians, pharmacists and medical technicians.
And also employ the best possible football officials.
Three weeks into a season with replacement officials, the league has been telling itself and anyone lending an ear that the folks in stripes don't matter. That's an intelligence-insulting lie.
The amateurs are as qualified as the regular officials now locked out, they say. That's a transparent lie, a giant fib, with greed at its core.
We long ago realized there are no limits to how far the NFL will go to
But how far will the NFL take this labor impasse? And how long are players and coaches and fans supposed to accept this?
Do the suits in the league's New York office really want to vigilantly impose fines on players wearing their socks too high or too low while ignoring the anarchy taking place on each play?
Officiating in any sport always is an imperfect practice, not unlike teaching or nursing or law enforcement, but replacement NFL officials are a hazard to the health of those suiting up. The subject is discussed after every game. These refs are shockingly inconsistent, missing too many calls, throwing flags when they should hold them and holding them when they should throw them.
Quarterbacks who slide to a stop are not to be hit, yet the 49ers' Alex Smith did exactly that last week and still took a whack from the forearm of Detroit safety John Wendling. Smith's nose was lacerated. No flag was thrown; Wendling later was fined $7,875.
This incident was a half-inch away from being a broken nose or a concussion or a serious neck injury.
Players and coaches clearly do not respect these refs, treating them like undertrained mall cops masquerading as big-leaguers. Did you see Patriots coach Bill Belichick grab an official after the game Sunday?
Some players are taking advantage, with rules violations they know normally would bring penalties or ejections.
These officials might be solid citizens, but amateurs have no place on the professional stage. It mocks the game, cheapens the product.
Mundy eventually will be fined. The NFL's video cops will study the video, see the hit, discover the infraction and lift a piece of his salary. When helmet hits helmet, there is a price to pay.
That won't help Heyward-Bey, who was strapped to a gurney and carted away from the Coliseum. When he raised his right arm on the way out, drawing cheers from the fans and clapping from Steelers assistant coaches in the upstairs box, there was a gigantic sigh of relief.
DHB spent Sunday night under observation at Eden Medical Center and was released Monday morning. He is expected to make a full recovery.
A flag also would not have done much for DHB, though a personal foul would have given the Raiders a first down much closer to the goal line.
What a flag might have done, though, was prevent the concussion suffered by Raiders tight end Brandon Myers. A few minutes after Heyward-Bey left the stadium, Mundy launched himself, helmet first, into the helmet of Myers. Again, no flag was thrown.
Flags are the official's tool to maintain control and penalize violators. They are signals of deterrence, to help keep the peace and restore order.
Replacement officials are going to cost some team a game -- just ask Green Bay. If the league doesn't fix this matter, fast, these amateurs could cost some athlete a lot more than a game.