Did you know that the Mariners have the lowest bullpen ERA in the AL?

AL Bullpens by ERA Rank Team ERA 1 Mariners 2.64 2 Red Sox 2.96 3 Athletics 2.97

Well, whatever. Bullpens don't pitch that many innings; it's probably just defense and sequencing luck. If you use a fielding-independent metric like FIP -

AL Bullpens by FIP Rank Team FIP 1 Mariners 3.20 2 Red Sox 3.33 3 Athletics 3.38

Wait, really? Well, uh, look at their home run rates. They're all super low! If you regress for that,

AL Bullpens by xFIP Rank Team xFIP 1 Mariners 3.37 2 Yankees 3.41 3 Red Sox 3.45

...oh.

To wit: ladies and gentlemen, your Seattle Mariners have the best bullpen in the American League. As we all know, the 2014 M's strength so far has been their pitching - and while Felix Hernandez's absurd dominance, Roenis Elias' surprising emergence, and Chris Young's magical powers have played big parts in that, it's certainly helped that the guys who come in after those other guys are tired have combined to pitch like an ace.

One specific ace, as a matter of fact.

K/9 BB/9 HR/9 GB% 2014 Mariners Bullpen 9.00 3.45 0.57 46.3% 2012-13 Gio Gonzalez 9.09 3.46 0.59 46.0%

As impressive as it is to compare the M's relievers to a guy who's been one of the fifteen best starting pitchers in baseball over the last two years, it's perhaps even more impressive to consider the guys who aren't in the Mariners' relief corps. These include:

Brandon Maurer, here for the moment but likely gone as soon as Taijuan Walker returns. 23-year-old converted starter who found 99 MPH velocity in the 'pen. Draws comparisons to Luke Hochevar, another failed starter who added velocity in relief and became a terrific setup man. Recently embarrassed David Ortiz so badly that Ortiz felt the need to start a staring contest as he walked back to the dugout. Has a 93 MPH cutter. Who the fuck has a 93 MPH cutter? That's two and a half standard deviations above the mean for cutter velocity. That's a Kenley Jansen cutter. Not good enough to get him into this bullpen, though.

bullpen, though. Carson Smith, top-tier relief prospect who posted dominant numbers in High Desert (!) and Jackson before losing some K's this year in Tacoma. Reportedly nigh-impossible to elevate the ball against; has a ground ball rate two and a half standard deviations above the AAA mean. In another bullpen, someone struggles, or gets hurt, and then Carson Smith gets a chance. Not in Seattle.

Stephen Pryor, currently languishing in AAA after returning from a shoulder injury. Was once a highly touted relief prospect with elite fastball velocity and rise. Struggled in his debut season, then was much better, then was much less healthy. Was expected to have a job waiting for him when he came back, on account of the absurdly good fastball. Noooope.

Stephen Kohlscheen, just promoted to AAA, presumably because the hitters in AA were accusing him of cheating. In Jackson this year he put up a K/BB ratio of over eight. Not a big ground ball guy, but that's OK when you strike out more than a man per inning without walking anyone. If he worked for any other franchise, Kohlscheen would be one of the first in line for a bullpen callup. Here, he is fourth.

A long reliever of any kind. Many clubs carry a spot starter like Chris Capuano or Hector Noesi to eat innings when the starter gets blown up. Not the Mariners - they have to get a couple innings each from three or four of their guys every time that happens. Somehow, it has yet to cause a problem.

There are more. Logan Bawcom has struggled lately, but began the year as Tacoma's closer. Lucas Luetge has had success as a major league LOOGY and could get a shot if Joe Beimel ever runs out of ACME brand anti-regression ointment. Todd Coffey and Logan Kensing are the kind of relievers that usually make their way onto the Mariners by midseason due to some combination of injury and dinger-proneness. Not this year.

Perhaps best of all, there isn't any particular "hidden stat" that can explain away these guys' success. Their BABIP is a little low at .281, but that's not a giant red flag. The same can be said of their 80% LOB%. Mike Zunino's catching helps, but it's not everything; since I called Dominic Leone out as a guy getting way too many called strikes he's upped his whiff game and really improved his K and BB rates. Indeed, if there's any one "unsustainable" stat I'd point to, it's that over the last month, Medina, Wilhelmsen and Rodney (the bullpen's three Wild Children) have allowed BB rates of 1.86/9 IP, 2.35/9 IP, and 1.54/9 IP. When all your "stuff" guys find their command at once, this is what you get: a month of ridiculous dominance.

The Mariners' bullpen is presumably going to get worse, because that's what bullpens do. They're super volatile. If the Mariners want to extract maximum value from their 'pen, the best way to do that probably involves selling high on either Medina or Leone - not that those guys are especially likely to regress, just that as the youngest and most controllable players they are also the most valuable on the trade market. Contending teams generally don't make a point of dealing away contributing young players, but the Mariners have such ridiculous right-handed bullpen depth that losing either Medina or Leone wouldn't be that big of a hit. And young, good, controllable, healthy relievers are pretty desirable assets. The Mariners shouldn't repeat the Oliver Perez mistake and hold on to a player for whom there is a bidding war just because they think he's helping out.

On the other hand, if the Mariners do decide to ship a reliever out of town, I'll be sorry to see him go. These have been a pretty fun few weeks.