What is an "inside-the-park grand slam"? How common is it in Major League Baseball?

A grand slam is when all 3 bases have a runner on them and the batter gets a hit and all 4 runners score because of the hit. Grand slams are not too common and almost all of them happen as a result of the batter hitting the ball over the fence for an out-of-the-park home run. An in-the-park home run is when the ball does not go over the fence but the batter is able to round all of the bases and score. So combining the two, you would have 4 batters scoring with the ball in play with the possibility of a runner being tagged out. Most often, with MLB there would have to be an error on the play or an unusual bounce.

Of the 154,483 home runs hit between 1951 and 2000, only 975 (0.63%; about one per 158) were inside-the-park. The percentage has dwindled since the increase in emphasis on power hitting, which began in the 1920s. While Jesse Burkett, who played in the major leagues from 1890 to 1905, had 55 career inside-the-park home runs, the leader in the live-ball era is Willie Wilson, who played in the major leagues from 1976 to 1994, and hit 13 inside-the-park home runs.

https://en.wikipedia.org/wiki/Inside-the-park_home_run

There have been 225 inside-the-park grand-slam home runs dating back to 1881 in Major League Baseball. It amounts to about 1.6 such scampers per season. For comparison, there have been 304 no-hitters pitched in MLB games since 1876. Even no-hit games are more common.

https://baseballscouter.com/inside-the-park-grand-slam/#google_vignette

The most recent one was on July 22, 2022! The majority of the inside-the-park grand slams occurred in the early days of baseball.

Blue Jays center fielder Raimel Tapia hit a rare inside-the-park grand slam on Friday night in Toronto's historic 28-5 rout of the Boston Red Sox at Fenway Park.

https://www.usatoday.com/story/sports/mlb/2022/07/22/blue-jays-raimel-tapia-inside-park-grand-slam-red-sox/10133002002/