The Chicago Cubs’ April was insane, frankly. Through 22 games, they went 17-5 (first team since the 2010 Rays to be that good or better, and before those Rays, it had been since 2003), outscored their opponents by 79 runs (second-best run differential over the first 22 in over 100 years, trailing only the 2003 Yankees; the fourth team in the last decade to outscore opponents by so much over any 22-game stretch), and were on pace to cruise past the all-time record for team walks. Oh, and at 6.18 runs per game, they were on pace to score over 1,000 runs, which would put them in the company of the 1999 Indians, the only team to score that many since MLB became fully integrated.

You probably knew all of that, though, and more to the point, we know none of that will keep up. The Cubs played a very weak April schedule. They got some key hits and strong overall performance from the likes of Matt Szczur and David Ross. They lost Kyle Schwarber for the season and Miguel Montero for at least a couple weeks. Jason Heyward has not made the hoped-for changes to his offensive game, remaining instead a patient hitter capable of hitting the ball hard, but not of getting it off the ground often enough to tap into the full power of that contact. Dexter Fowler played out of his mind for two weeks, but while he’s a better player than the (ahem) market decided he was this winter, he’s still Dexter Fowler. The Cubs aren’t a 110-win team. I’m not sure I would peg their final record any higher today than I would have on Opening Day, all things weighed and accounted for.

Instead of gushing over one awesome month, then, let’s do something a little more substantial. Let’s gush over two crazy months. Specifically, let’s gush over the fact that the Cubs’ Park Adjusted Defensive Efficiency (PADE) since September 1 of last season is off the friggin’ charts. A quick primer, for those who don’t spend as much time as I do in the ‘Statistics’ tab on our fine site: Defensive Efficiency is our way of measuring team-level defensive work. It’s extremely simple, almost “1-(BABIP allowed)” simple, but basically, it gives the frequency at which teams turn opponents’ balls in play into outs. For instance, the Mets had a league-worst 0.665 Defensive Efficiency Rating for April, which you can (roughly) read to mean that they got about two-thirds of an out for each ball in play their pitchers allowed.

PADE, as the name suggests, accounts for the way ballparks affect expected outcomes on balls in play (even a groundball in Colorado isn’t the same thing as one in San Diego), and then it gives a slightly different number. It usually comes out as a small single-digit number, plus a few decimal places, centered around zero. Accounting for park factors, the Giants actually had the league’s worst defense in April, with a -4.53 PADE. Essentially, they got 4.53 percent fewer outs on the balls in play they allowed than we would have expected, given average defense.

That brings us to the Cubs, and to the somewhat startling stretch they’ve had defensively. Last October, I asked for a report showing all 30 teams’ PADE figures for each month of the season. Pooling those with the 30 one-month samples we have so far this year, I have 210 team months of PADE. Of those, the highest figure by far is the Cubs’ September and October last year, at 8.14. The second-highest, though, and still with some elbow room over third place, is the Cubs’ 6.95 mark this April. Over their last two months of baseball (a tidy 54 games, a third of a full season), the Cubs have posted a PADE of 7.66. For some perspective, only three teams in the 65 years for which we have this stat have ever eclipsed a 4.60 PADE for a full season. Only 11 teams have ever had a PADE of at least 3.83 (or, half what the Cubs have posted over these last 54 games).

I’m hesitant both to ascribe meaning to this (any statistic with substantial adjustment factors might suffer from a lot of noise in a short sample, which is still what we’re talking about when we talk about 54 games) and to hazard a guess as to whether it’s sustainable. All I really want to do today is highlight the trend, and to quickly discuss a couple contributing factors thereto.

Firstly, there’s the pitching. We’ve established, by now, that Jake Arrieta just never gives up hard contact. He’s allowed the lowest average exit velocity among qualifying starters since the dawn of Statcast. As it turns out, the entire Cubs staff is pretty good in this regard, as the team has the fourth-lowest average exit velocity allowed since last September 1. They also allowed only 199 batted balls at least 100 mph off the bat over that stretch, which was not only the fewest in the majors, but 34 fewer than the next team on the list (the Mets). (Take home runs out of that equation, and the number of balls hit that hard drops to 172, also the fewest in the league, this time 26 fewer than the second-best Angels.) Avoiding hard contact is a vital step toward racking up outs on balls in play, and the Cubs seem to be nearly the best at it. It also helps, when Fowler and Jorge Soler make up two thirds of the starting outfield, to keep the ball on the ground, and Cubs hurlers have the best ground ball rate in baseball through the first month.

That doesn’t explain this away, though. On batted balls in play under 100 miles per hour, since September 1, the Cubs have the lowest batting average allowed, at .217. (They’re almost exactly in the middle of the pack, by the way, in batting average allowed on balls over 100 mph, whether you include homers or not.) They’re not doing it by shifting a ton. In fact, they’re not among the top half of the league in shifts deployed. Rather, they seem to be leaning on their collection of some very, very good young athletes who can do it all defensively (Addison Russell, Jason Heyward, Javier Baez, Kris Bryant, and Anthony Rizzo) and a few guys who make up for limited range with sure-handedness and good baseball IQs (Fowler, Ben Zobrist, Tommy La Stella). When one considers the pitching staff’s ability to avoid hard contact, it’s easy to see how the team could stand to give up the superior quickness and range of Starlin Castro to add Zobrist’s bat and steady hands, or how Joe Maddon can confidently pencil in La Stella and his second baseman’s reaction time at the hot corner sometimes, so as to keep his potent left-handed bat in the lineup.

One other thing I think needs to be said, an eye-test kind of thing that you can ignore if you want to: this team hunts outs like few others I’ve seen. The two primary catchers are two of the best in the game at throwing behind runners to get pickoffs, sometimes at huge moments. Rizzo’s famous catch on the tarp at Wrigley Field last year is one example of the lengths to which he’ll go to steal an out. He’s as good at using his arm to do so as he is at using his glove. Baez is a defensive freak, probably the best defender the Cubs have at both shortstop and third base. In the video I linked to a sentence back, Baez applies an unbelievably quick tag, basically by allowing Starling Marte to come between the baseball and every part of him except his glove, which catches the ball and touches Marte almost at the same time. Two innings after that play, with runners on the corners and nobody out, Baez was playing back, the Cubs willing to trade the tying run for a double play. When Josh Harrison bounced a ball that Baez had to charge a bit, though, Baez decided to fire home. He had to get rid of it fast, no time to gather himself, but he threw it 95 miles per hour and preserved the Cubs’ lead. Baez, Russell, Rizzo, Heyward, and Ross blend creativity and aggressiveness into their defensive skill sets, in a way that creates opportunities and can stifle rallies for opposing teams.

Are the Cubs going to hold opposing hitters to a .256 BABIP for the rest of the season, as they have over the last two months’ worth of games? That seems impossible. It’s not technically unprecedented, but only five teams have held opponents to BABIPs south of .260 in the last 40 years, and the most recent was the 1990 Athletics. The 2001 Mariners (.262) are the closest thing to a truly contemporary full-season match for what the Cubs are doing, and with league-wide BABIP more or less at its historical peak, comparisons to teams any further back than those Mariners don’t really make sense.

It’s a good bet that the Cubs’ combination of raw defensive talent, pitchers who create weak contact, the depth to keep fresh legs and eyes in the game all the time, and some especially tenacious playmakers make them genuinely great at preventing hits on balls in play. It’s an even better bet that there’s some regression coming, so if the Cubs want to try to match the 2001 Mariners in a less obscure historical sense (or even to hold off the Pirates and Cardinals in the NL Central), they’ll need to keep scoring a lot of runs, and not worry about (inevitably) allowing a few more than they have so far.