On 6,105 occasions last season, a major leaguer walked to the plate and hammered a baseball over the outfield wall. The 2017 season broke the home run record that was set in 2000 — the peak of the steroid era — when players hit 5,693 homers, and it built upon the remarkable 5,610 that were hit in 2016. It was a stunning display of power that played out in every MLB park almost every night. And with spring training underway in Florida and Arizona, MLB’s power surge is showing no sign of letting up.

But while we now know what caused the spike in home runs at the turn of the century — even if we didn’t at the time — the reason for the most recent flurry of long balls remains an unsolved mystery. Any number of factors might have contributed to the home run surge, including bigger, stronger players or a new emphasis on hitting fly balls. But none of those possibilities looms larger than the ball itself.

MLB and its commissioner, Rob Manfred, have repeatedly denied rumors that the ball has been altered in any way — or “juiced” — to generate more homers. But a large and growing body of research shows that, beginning in the middle of the 2015 season, the MLB baseball began to fly further. And new research commissioned by “ESPN Sport Science,” a show that breaks down the science of sports, suggests that MLB baseballs used after the 2015 All-Star Game were subtly but consistently different than older baseballs. The research, performed by the Keck School of Medicine at the University of Southern California and Kent State University’s Department of Chemistry and Biochemistry, reveals changes in the density and chemical composition of the baseball’s core — and provides our first glimpse inside the newer baseballs.

Looking inside the balls and testing their chemical composition revealed that the cores of recent balls were somewhat less dense than the cores of balls used before the 2015 All-Star Game. The newer cores weigh about a half a gram less than the older ones, which might be enough to cause baseballs hit on a typical home run trajectory to fly about 6 inches farther. That alone is hardly enough to explain the home run surge of recent seasons, but when combined with previous research finding that baseballs began to change in other small ways starting around the same time, it suggests that a number of minor differences may have combined to contribute to the remarkable upswing in home run power we’ve witnessed since 2015.

Asked about these findings, MLB noted that it had commissioned a group of scientists and statisticians to investigate any changes to the ball, and that the committee would issue a report on its research soon. According to Alan Nathan, one of the physicists on the commission, the task force found that all the characteristics that MLB regularly measures, including the weight, circumference, seam height and bounciness of the ball, were within ranges that meant variations in the baseballs were unlikely to significantly affect home run rates. MLB declined to provide the data supporting these assertions.

Independent investigations by FiveThirtyEight, publications like The Ringer, and Nathan himself have shown differences in the characteristics of the ball and the way it performs. Research has shown that balls used in games after the 2015 All-Star Game were bouncier and less air resistant compared with baseballs from the 2014 season, when players hit a relatively modest 4,186 homers, the fewest since 1995. (Nathan noted that MLB does not regularly measure air resistance.) Taken together, these changes would result in a ball that would come off the bat at a higher speed and carry farther. While investigations have been able to show that the baseball behaves differently in recent years, no one had looked inside the ball to see if there was evidence of changes to the way the baseball is constructed.

So far, these investigations have primarily looked at the exterior of the baseball. Broadly, MLB baseballs — which are produced by Rawlings in Costa Rica — are made of three components: an exterior shell of cowhide, a winding of several layers of yarn, and a core of rubber-coated cork, also known as a “pill.” To analyze possible changes to the inside of the ball, particularly the core, “ESPN Sport Science” purchased one new ball from Rawlings and seven game-used baseballs from eBay, confirming their authenticity through MLB’s authenticator program. The eight baseballs we tested were split into two groups: an “old group” of four balls used in games played between August 2014 and May 2015, and a “new group” of three balls used in games played between August 2016 and July 2017, plus the brand-new ball. The aim was to see if the internal composition of the baseballs had changed in ways that would affect the ball’s performance. The balls were first analyzed by Dr. Meng Law, Dr. Jay Acharya and Darryl Hwang at the Keck School of Medicine at USC using a computerized tomography, or CT, scan. This test is typically used to look inside a human head or body, but in this case, it allowed Dr. Law’s team to examine the interior of the baseballs without cracking them open and destroying them. Initial CT imaging showed that baseballs in the same group had a negligible variation in internal properties. When comparing the new and old groups, however, there was a clear difference in the density of the core. In an MLB baseball, the core consists of four parts: a cork pellet at the center, surrounded by a layer of black rubber held together by a rubber ring where the halves meet, all of which is then molded together in a layer of pink rubber. Dr. Law’s team isolated the density difference to the outer (pink) layer of the core, which was, on average, about 40 percent less dense in the new group of balls. While other parts of the ball showed slight differences in density and volume, none were as noteworthy as the changes to the core.

It’s not just that the inside of the ball looks different — the chemical composition of the cores appears to have changed as well. After being tested at the Keck School, the same set of balls were sent to Kent State University. There, researchers at Soumitra Basu’s lab in the Chemistry and Biochemistry department cut open the balls to examine the cores using a thermogravimetric analysis (TGA). This test essentially cooks a material to see which parts parts of it vaporize at which temperatures. Using that information, researchers can create a molecular profile of a given material.

This test showed that the pink layer of the core in baseballs from the new group was, on average, composed of about 7 percent more polymer than the same area in baseballs from the old group. Additionally, an analysis with a scanning electron microscope showed that the same layer in the new balls contained, on average, 10 percent less silicon, relative to the amount of other ingredients in the pill. According to the Kent State researchers, these chemical changes produced a more porous, less dense layer of rubber — which explains the results seen in the CT scan at the Keck School.

It may not seem obvious, but these slight changes in the chemical composition of the core could have an impact on how the balls played once they were sewn up and shipped to major league teams. Less dense cores could mean lighter baseballs. The cores of the new balls weighed, on average, about 0.5 grams less than the cores from the old group. This difference was statistically significant, which means it’s highly unlikely that it was due to sampling error. The overall weight of the balls also dropped by an average of about a 0.5 grams between groups, but, unlike with the cores, this difference was not statistically significant.

Half a gram isn’t much — it’s only about the weight of a paperclip. A tiny change like this might add only about 6 inches to flight of a baseball hit on a typical home run trajectory, according to Nathan’s calculations. But the timing of these changes to the weight and density of the core coincides with a much larger boost to the bounciness of the baseball. According to a previous analysis performed by The Ringer, that increase in bounciness alone would add around 0.6 mph to the speed of the ball as it leaves the bat and add roughly 3 feet to the travel distance of a fly ball — enough to make the difference between the warning track and the stands.

On top of the fact that the balls became bouncier as the core itself changed, previous research at FiveThirtyEight showed that they also became less air resistant. The decrease in drag is probably a result of a smaller, slicker baseball with lower seams. The change in air resistance could add an additional 5 feet to the travel distance of a fly ball. Combine all these factors together — a lighter, more compact baseball with tighter seams and more bounce — and the ball could fly as much as 8.6 feet farther. According to Nathan’s calculations, this would lead to a more than 25 percent increase in the number of home runs. Asked whether these changes in combination could have significantly affected the home run rate, MLB declined to comment.

In actuality, home runs spiked by about 46 percent between 2014 and 2017, which means that the changes to the ball could account for more than half of the increase. The remainder could be reasonably chalked up to a philosophical shift among MLB hitters, who are likely swinging upward to maximize the number of balls they hit in the air and are not shy about the increase in strikeouts that may come with that approach.

MLB Commissioner Rob Manfred has repeatedly denied that the baseball is juiced. On numerous occasions, he has said league testing found that baseballs continue to fall within the range that MLB designates as acceptable, and he recently said that MLB testing showed the balls to be fundamentally the same. But even if the baseballs still meet the league’s manufacturing guidelines, their performance could change enough to double (or, theoretically, halve) the number of home runs hit in a year.

In fact, in January of 2015, Rawlings filed a patent application for a manufacturing process that would allow it to produce softballs and non-MLB baseballs that were as bouncy as possible while still falling within the manufacturing specifications set by the league. This type of ball is constructed quite differently from MLB baseballs, so there’s no indication that this patent means Rawlings is deliberately manipulating major league baseballs in this way, but it demonstrates that it’s at least theoretically possible for balls to be “fundamentally the same” while also performing differently than they have in the past.

Kathy Smith-Stephens, senior director of quality and compliance at Rawlings, said that no change had been made to the baseballs but that “natural variation” occurs in the manufacturing process. She noted that they “continuously tweak” — though later in the interview she asked that we say “continuously refine” — the manufacturing process in an effort to reduce variations, but said that Rawlings’ internal testing had shown no difference in the ball’s weight or bounciness.

Evidence that the baseball is at least partially responsible for the last few years’ spike in the home run rate mounted throughout the summer of 2017 and reached a peak during October’s World Series. In those seven games, the Houston Astros and Los Angeles Dodgers smashed 24 homers, including eight in one game. In the wake of this power display, Manfred asked all 30 teams to start storing baseballs in a climate-controlled room and commissioned a task force of scientists and statisticians to investigate whether the ball was juiced in 2017. Our own research, combined with controlled tests from three separate academic laboratories, strongly suggests that the physical properties of the ball have changed. Taken together, all these studies give us a lot of evidence to suggest that today’s baseballs differ in meaningful ways from those of a few years ago. In other words, there are many questions for Manfred’s committee to address.

Special thanks to Sean O’Rourke, Dr. Cynthia Bir and Nathan Beals for additional research assistance.