One of the biggest surprises of this season has been the return of the home run.

Testing for performance-enhancing drugs, a more consistent calling of the strike zone and better pitching — particularly the preponderance of hard-throwing relievers — were supposed to mean the end of the longball.

Yet, there 5,610 home runs hit in the regular season, the second-most in major league history, behind the 5,693 hit in 2000 during the height of the Steroid Era.

Just two seasons ago in 2014, 4,186 homers were hit. That is a 34 percent increase.

Some players around the major leagues have offered the theory that the composition of the baseballs is harder than recent years. However, Commissioner Rob Manfred said that is not the case after testing the baseballs.

“We are absolutely convinced that this issue is not driven by a difference in the baseball,” Manfred said. “My own view is that the spike is related to the way that the game is being played now, the way that we are training hitters from a very young age, and we have not been able to find any external cause that explains the spike in home runs.”