Kyle Seager has been one of baseball's best-kept secrets for three and a half years now. He plays in the northwest corner of the country, one that's represented by just a single baseball team, one that team that hasn't been anything close to good up until last season. Of course, the Mariners finally became nationally relevant in 2014, but Kyle Seager has been nationally relevant since his first full big league campaign back in 2012. And while hardly anyone outside of the Pacific Northwest noticed, the Mariners certainly understand what they have in Seager as they just locked him up through his age-33 season for a total of $100 million. It's probably the biggest extension no one saw coming, yet we probably should have.

Seager was drafted back in 2009 out of the University of North Carolina, a storied collegiate program. He didn't suffer from a lack of exposure as his team made it to the National Championship Series when he was a freshman starter (although they would go on to lose to Oregon State for the second time in as many years). Two seasons later, it was Seager's teammate, Dustin Ackley, who was the most highly-regarded Tarheel and the M's popped him second overall in the 2009 draft. Seager stayed on the board a little longer, going to Seattle in the 3rd round, 97th overall. As Dave Cameron pointed out yesterday, Seager wasn't thought highly of, even for a third-rounder. Baseball America noted that he was viewed as a "grinder" and a low-floor/limited ceiling player with below average power potential. The Mariners took him anyway and he jumped right into full season ball, hitting reasonably well after a long summer of college baseball.

That offseason, no one really took notice and Baseball Prospectus left him off their 2010 Mariners Top Prospect List. Seager went on to destroy the California League the next season, putting up a wRC+ of 143 over 135 games, hitting 14 homers, stealing 13 bags and walking nearly as often as he struck out while playing second base. The performance was hard to ignore and he cracked the BP Top 10 that offseason, ranking 8th in the Mariners' system. Still, he was seen as a questionable hitter and a less-than-ideal defender at second.

Other than the bat, Seager's tools are a bit short. He has below-average power with little projection, and while he makes all of the plays on balls he gets to, he's not especially rangy at second base. Like most players who put up big numbers in the Cal League, scouts are reticent to put a big number on him until he proves it at a higher level.

Seager didn't slow down in 2012, hitting .312/.381/.459 over 66 games in AA. He moved to AAA, was even more potent in a 24-game stint there, then found himself in the majors at just 23-years old where he played 53 games, exhausting his prospect status. He hit modestly and wasn't a disaster in his debut with a wRC+ of 96, but he was far from a proclaimed future star. The Mariners had little to lose and installed Seager as their Opening Day third baseman in 2012 and he simply never looked back. Take a look at his line over his first four seasons:

G PA HR ISO BB% K% BABIP wRC+ UZR/150 WAR 2011 53 201 3 0.121 6.5% 17.9% 0.303 96 -1/8 0.7 2012 155 651 20 0.163 7.1% 16.9% 0.286 108 1/2 3.6 2013 160 695 22 0.166 9.8% 17.6% 0.290 115 -3.2 3.7 2014 159 654 25 0.183 8.0% 18.0% 0.296 126 9.2 5.5 Steamer 2015 143 614 20 0.172 8.6% 17.6% 0.292 120 n/a 4.1

Three years after being drafted as a limited upside guy and a Bill Mueller comp, Seager was a 20 home run hitter who produced offense at an above-average level while playing good defense at the hot corner in his first full big league season. His 2013 campaign was another step forward, and in 2014, he was simply outstanding.

You don't have to look very hard to notice the gradual improvement in his game as his offense has grown every year, and despite his 2013 defensive metrics, he should be viewed as an above-average defender. How far above average depends on how much you want to weight his defense, but clearly he's graded out well in that category to go along with his burgeoning offense. He's proven durable and incredibly consistent to boot. The Mariners gambled on a pair of UNC picks in 2009, but it is Seager, not Ackley, that's proven to be the standout.

But what was it that made the Mariners believers in Seager? After all, he was just given $100 million over seven years, so not only are they buying Seager's past, they're believers in his future. Looking at the standard age curve, it suggests that players' plate discipline and power increase as they hit their prime and continue to age. During the player's peak, typically somewhere between 27 and 30-years old, all of their tools should align before the player starts to slowly break down physically, loses a step, the bat slows and they eventually fade just like everyone else. Seager's plate discipline, which was already good, has held steady through his age-26 season. His power, predictably, has grown each season and the Mariners are clearly betting that it at least holds steady if not takes one last step forward.

But part of what makes his performance believable is his BABIP. Looking at his career numbers, it's held steady at a league-average level, or perhaps a touch below, which makes sense given Seager's speed. His overall production is free of random BABIP spikes, meaning that it looks incredibly stable. Should he have a season where he experiences some "good luck," his impact could explode. In short, Kyle Seager has been as steady as they come in three full seasons in the majors, making his production remarkably believable.

And taking a look around the league this winter, one in which Pablo Sandoval has netted big money as a similar player, Seager is producing offense at a similar rate and playing perhaps better defense than the Panda. He's also been more durable, more consistent and is yet making less annual money than Sandoval. Yes, he's a year younger, but Sandoval is getting nearly $6 million more per season. This marks a kind of bargain for the M's as they're getting a younger player, a more durable player and someone with comparable offensive production at a cheaper rate for a longer period of time. This is due in part to the fact that Seattle had team control over Seager for another two seasons.

Over the life of his deal, Seager should earn about $14.25 million per season on average (how Seattle chooses to spread that money out may not look like a straight salary as these deals are usually backloaded). At the going rate for free agent wins, that's essentially the cost of two wins. One could argue that wins are slightly cheaper than that now, but as baseball salaries inflate, two wins could cost more than that down the road. Calling it the cost of two wins is a pretty safe bet, yet Seager projects as more of a 4-win player for the next few years before his decline phase slowly sets in. It's not unreasonable to project Seager for 23-27 wins over the life of the deal, production that would cost over $180 million dollars on the free agent market. Put it together and this looks like an opportunity for Seattle to extract some serious surplus value from a homegrown star.

Kyle Seager, despite his production, still remains somewhat under the radar, but that narrative is starting to run dry. He's among the best at position in the game and just got paid in a corresponding fashion. Despite his draft stock, prospect status and uncoupling debut, Kyle Seager didn't look like a player who would ever net himself a $100 million deal. But once establishing himself, he's done nothing but produced with incredible consistency by the way of peripherals that are incredibly sustainable. The Mariners surely took notice and officially put Kyle Seager on the map, where he surely belongs.

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Jeff Wiser is an editor emeritus at Beyond the Box Score and co-author of Inside the 'Zona, an analytical look at the Arizona Diamondbacks. He and Ryan Morrison are the hosts of The Pool Shot Podcast and you can find his work on craft beer at BeerGraphs. Follow him on Twitter @OutfieldGrass24.