It's no secret offense around baseball has been declining the last few seasons. MLB teams hit .251/.314/.386 on average in 2014 compared to a .262/.333/.418 league average in 2009. In just five years the league lost 51 points of OPS. That's huge.

There is no shortage of explanations why offense is going down, and it's not just one thing. Widespread infield shifts, harder-throwing pitchers, specialized relievers, stricter performance-enhancing drug testing, all of that and more has contributed to baseball's league-wide decline in offense.

There's another reason why offense is on the way down: The strike zone is getting larger and larger each year, particularly the bottom of the zone. In a must-read piece at The Hardball Times, Jon Roegele analyzed the strike zone using PitchFX and found that the average strike zone in 2014 measured 475 square inches compared to 435 square inches in 2009. That's a huge increase!

Here are two images from Roegele's post showing the difference between the average 2009 and 2014 strike zones. The front plane of the plate has been broken up into one inch squares -- any square with more called strikes than balls was included in the strike zone -- and the 0.0 marker on the horizontal axis is the middle of the plate. The images are from the umpire's point of view with 2009 on the left and 2014 on the right.

As you can see, almost all of the growth has occurred at the bottom of the zone. Teams know this, too. Yankees pitching coach Larry Rothschild puts a string across the plate during spring training bullpen sessions to show his pitchers just how low they can go to get a called strike. From Joel Sherman of the New York Post:

In bullpen sessions this year, pitching coach Larry Rothschild has introduced a technique he used in previous locales, but not with the Yankees — he has a yellow string that crosses the bottom of the strike zone and he actually is encouraging his pupils to hit the string.

Rothschild said he does not believe most pitchers have a good idea of just how low down they could go and get a strike, and this provides a visualization. He added it also helps with the fundamental of finishing pitches with the right mechanics — full-extension follow-through — necessary to drive the ball to the lower part of the strike zone consistently.

According to Roegele, 16.8 percent of pitches were thrown 18 to 24 inches off the ground (approximately the bottom of the zone) back in 2009. This past season it was 18.3 percent. Hitters swung at 45.0 percent of these pitches in 2009 and 49.2 percent in 2014 because they know it is more likely to be called a strike these days.

Pitching to the bottom of the zone is something pitchers have done for decades. It's tough to hit the ball down, and even when hitters do make contact, they tend to beat the ball into the ground. Pitchers like that. Nowadays pitchers can pitch lower in the zone than they have at any point in the last eight years (the PitchFX era) and it's sapping offense around the league.

Why is the strike zone growing downward? Beats me. The zone is clearly defined in the rulebook as knees to letters, though it's been more of an amorphous blob in recent years. If the zone continues to expand, offense is only is going to go down even further. Hopefully there's a correction sometime soon and the zone is called as it should be.