On June 24, 1997, those of us gathered at the Kingdome to see Seattle Mariner Randy Johnson fan 19 also may have witnessed the longest home run ever hit in Seattle. Had it instead been launched years later on a still, sunny day at Safeco Field, the ball might’ve cleared the roof of the Silver Cloud Hotel across Royal Brougham Way.

Mark McGwire’s shot to the heights of the left-field bleachers was one about which to tell your grandkids to tell their grandkids. Team officials nearly immediately claimed the poke, if not for the Dome’s abrupt concrete barrier, would’ve found its resting place 538 feet from home plate.

“Unbelievable,” marveled M’s broadcaster Dave Niehaus, whose belief systems had been challenged two years earlier when, upon witnessing the team beat the Yankees and reach the American League championship series, he exclaimed for the ages: “I don’t believe it.”

Neither did other arbiters when it came to the alleged length of the McGwire shot. Few who saw or heard the blast (my buddy later said, not inaccurately, that it sounded “like a car wreck”) would dispute that it was of Ruthian proportions. Within weeks, though, skeptics of the 538-foot “guesstimate,” as M’s officials later called it, had downsized the fly ball to perhaps a mere 460 feet: like Mickey Mantle in his prime.

One imagines, then, given the events of Monday, we’ll need to find something else from that day to tell the grandkids. McGwire is credited with 57 other home runs that season, all of them likely influenced by steroids.

The one-time slugger made and reiterated the admission Monday, perhaps as a means of coming out of the reclusive shell he’s inhabited since 2005, when, testifying during United States congressional hearings, he as good as admitted that he used performance-enhancing stuff during a 16-year career.

What, then, are we left with to impart to the grandchildren? Try this five years from now, when Randy Johnson, who retired Jan. 5, is unanimously voted into the Baseball Hall of Fame:

“I was there that day in June of ’97, when R.J. K-ed 19 against Oakland. The only really bad pitch was a 97-mile-an-hour fastball. Some guy, not exactly a Hall of Fame type, took it long. The M’s lost 4-1.”