As June headlines go, this was a stunner:

Bob Stoops is retiring at age 56.

Nobody saw it coming. Not at this time of year and not at this time of Stoops’ life. He appeared to be in good shape physically and his Oklahoma football program still was dominating the Big 12, having won the conference the past two years, even if it had slipped in terms of national prestige.

But there it was, real and true, first reported Wednesday afternoon by The Oklahoman: Stoops, the third-most-accomplished active coach in college football, is walking away.

A source with knowledge of the situation told Yahoo Sports that Stoops’ retirement and the elevation of Lincoln Riley to head coach had been in the works for “a while.” That source said the original target date to announce the move was Friday, but it became apparent that news of that gravity would not stay quiet for long. Thus the timetable was accelerated to Wednesday by The Oklahoman’s breaking news story.

As we await more explanation on this bombshell, this much is certain: Stoops didn’t have much left to prove to anyone and could leave on his own terms. He won a national title in 2000 and re-established Oklahoma to the level it had enjoyed under Bud Wilkinson and Barry Switzer. Through achievement and longevity and loyalty, Stoops became the third pillar of coaching greatness at the school.

Ultimately, his family history may have influenced his decision. Stoops’ father, Ron, a legendary high school coach in Ohio, collapsed on the field during a game and later died en route to the hospital at age 54. It’s the kind of life lesson that can stick with a man forever.

The quote in the Oklahoman story, from an unidentified source, indicated that nothing is amiss within the program or Stoops’ personal and professional life. It put a positive spin on Stoops’ decision to retire and turn the program over to touted 33-year-old offensive coordinator: “Great young coach, the program’s in great shape. Good recruiting class. Got a great young coach that anybody would want to have, ready to go. He’s as good a coach as there is in America, at this stage in his career.

“And Bob got to do it in his way. Doesn’t get any better than that.”

The only possible regret Stoops could have is the inability to maintain the stratospheric career arc of his early tenure at Oklahoma. He won a national championship in his second season and played for the title three times between the 2000-04 seasons, but something seemed to break in a shocking 55-19 beatdown for the title against USC in ’04. The Sooners were never again as powerful, although they did reach the 2008 BCS Championship Game before losing to Florida.

But the impact of Stoops’ retirement reverberates far beyond the state of Oklahoma. It has regional and national implications as well.

It’s another blow to the reeling Big 12, which has been leaking football clout for years. Already the definitive fifth wheel in a playoff structure built to reward four conferences per year, it also now has lost its only active coach who has won a national title, played for a national title or been to the College Football Playoff.

Mack Brown is gone, Art Briles is gone, and now so is Stoops. Bill Snyder is an amazing coach, but he’s also 77 years old. That’s not the guy to carry the Big 12’s banner into the future.

And consider this stark fact: There are only four active Division I head coaches who have won a national title. Nick Saban has five, Urban Meyer has three, Dabo Swinney and Jimbo Fisher each have one.

Where have all the giants of the game gone?

Some got old and/or stale (Brown, Steve Spurrier, Les Miles). Some turned pro (Pete Carroll, Chip Kelly, Jim Harbaugh – though Harbaugh is now back near the top of the college game). Some others who appeared ready to ascend have either imploded or regressed (Briles, Brian Kelly, Kevin Sumlin, Mark Dantonio, Mark Helfrich).

That’s a lot of churn at the top. There is a coaching power vacuum in college football right now, and it’s accentuated by Stoops’ sudden departure.

Who steps up?

At their present pace, Swinney and Fisher appear capable of long-term success and multiple national titles. Chris Petersen could be building a monster at Washington, and David Shaw is sustaining success at Stanford. Harbaugh and James Franklin appear poised to battle Meyer in the Big Ten for years to come.

But the days of doing what Stoops and Saban have done – repetitive winning without any kind of breakdown – could be even more fleeting going forward. Meyer, for all his excellence, suffered through a dark period that led to leaving Florida and starting over at Ohio State.

Stoops’ 14 seasons of double-digit wins in 18 years is a remarkable run. Perhaps most remarkable: It was all at one school.

Will any coach ever again last 18 years in one pressurized place like Stoops did? Doing it at Kansas State like Snyder, Iowa like Kirk Ferentz or TCU like Gary Patterson is one thing – there are no realistic expectations to win a national title there. Doing it at Oklahoma, where there is hardware from seven consensus national titles in the trophy case, is something else entirely.

The pressure to annually achieve Oklahoma-level success now falls to Riley. At 33, he’s just a baby, without head-coaching experience. But in 1999, the same school threw the keys to a 30-something coordinator and never lived to regret it.

Now Bob Stoops is 56 and suddenly, shockingly retired. He will be missed – in Oklahoma, around the Big 12 and across college football.

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