It started a little more than five years ago.
On Nov. 14, 2009, Jim Harbaugh's Stanford Cardinal were in the midst of pounding Pete Carroll's USC Trojans when the former, despite being up 27 points with less than seven minutes remaining, decided to go for two—setting off a powder keg that would eventually flourish into the NFL's fiercest rivalry.
Carroll famously responded with "what's your deal?" during one of the more tense postgame handshakes you'll see, but when he departed the collegiate ranks for the Seattle Seahawks just two months later, it seemed as though the budding rivalry wouldn't see any subsequent chapters.
Instead, it has turned into a full-fledged book.
Nearly an exact year after Carroll signed with the 'Hawks, Harbaugh used his Orange Bowl success to vault to the NFL, taking a job inside the same division with the San Francisco 49ers.
Harbaugh enjoyed immediate success in 2011, taking the 'Niners to a 13-3 record and a spot in the NFC Championship, walking over Carroll's Seahawks, who struggled to a 7-9 record, in the process.
But in 2012, the rivalry took off.
With both teams sitting at 4-2 and in position to grab an early stranglehold on the NFC West, San Francisco nabbed its fourth straight victory over the 'Hawks with a 13-6 win at Candlestick. Neither quarterback hit the 150-yard passing mark, as the game turned into a physical battle on the ground, epitomizing what both teams had become under their fiery head coaches.
"That was the most physical 30 minutes of football in the second half that I have ever seen our football team play," Harbaugh said after the game, via ESPN.com.
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At that point, the former Stanford coach was 3-0 over Mr. What's Your Deal in the NFL, but Carroll's squad was only getting better. Following that loss, the 'Hawks went 12-3 over their next 15 games, spanning the end of 2012 and the start of 2013, including two routs over the 'Niners by a combined score of 72-16.
The second of those had many pundits anointing Seattle as the best team in the league, and as they continued to reel off impressive wins, it was difficult to argue.
In customary fashion over the past two seasons, though, as one squad established itself as the cream of the crop in the NFC west, the other only raised its game to a new level. Since dynamic wide receiver Michael Crabtree has returned to the field, the 'Niners have reeled off seven straight wins, including a 19-17 triumph over Seattle in Week 14.
And now they'll meet again with a spot in the Super Bowl on the line—and with the exception of fans in New Orleans, fans in Carolina and liars, most will admit the NFC Championship shouldn't be settled any other way.
For as much as grown adults in San Francisco and Seattle turn into little kids whenever talking about—or yelling at—the other, their beloved teams are essentially mirror images.
One has a brash head coach, one has a pompous head coach and both have one who no one else in the country can stand. Both have hard-nosed running backs that are allergic to tackles. Both have quarterbacks capable of beating you through the air and on the ground. Both have elite, physical defenses. One has a wide receiver who loves to talk, the other has a cornerback who loves to talk and both back it up on the field.
The elite talent, the similarities in style, the history of the head coaches, the refusal to sell tickets to the other fanbase, the unadulterated hatred—everything about this matchup makes it one of the most fascinating rivalries of quite some time.
One team may advance on Sunday, but that will hardly do anything to slow down the rivalry—and that's a good thing.