Baseball is a vast chamber of oddities, and among them is the "immaculate inning," which I first stumbled upon recently as I read about Cubs pitcher Rich Harden. An immaculate inning occurs when a pitcher strikes out three batters in nine pitches. It is a surprisingly rare occurrence in the history of professional baseball. Only three pitchers have done it twice, and no pitcher has done it three times. It happened once earlier this year, when AJ Burnett faced the Marlins, and a number of other times this decade, as the phenomenon has become increasingly common.
To demonstrate the change in frequency over time, I made this chart showing immaculate innings pitched in the MLB by decade since 1880:
As one friend pointed out, the best explanation for the increase in recent decades appears to be the advent of the modern reliever, especially the flame-throwing, one inning closer (more immaculate innings have been thrown in the 9th inning* (eight) than in any other inning), though starters--such as Burnett--have also been throwing them with impressive frequency. Additionally, a lot more teams play today, meaning more innings pitched and more opportunities for immaculate innings.
* To be sure, two of those eight innings were thrown by starters who threw complete games--talk about an impressive way to finish the day.