Showalter, a founding franchise builder and manager for the first three years of Arizona's Major League Baseball existence, said before the game that in all his years here he had never taken a dunk in the pool located beyond the low right-center field fence where, coincidentally, Eaton's homer landed.
"When I first got here I think they had an interior fence," Showalter recalled. "It was about 390 out there. We wound up taking that out. In Arizona you have to find some places to let pitchers pitch to. That worked out pretty well. It's not as short as it was. We added the swimming pool. I haven't been out in the pool yet."
There were five homers in the game, three by the D-backs and two by the O's.
The Orioles got homers from Chris Davis -- his Major League-leading 43rd to tie the score at 5 in the eighth -- and catcher Matt Wieters, whose sixth-inning drive landed in the boxes above the yellow line in right-center field some 413 feet away.
Asked if the ball flies out of the ballpark the way he remembered it back in his day, Showalter quipped afterward:
"I'm 57 years old. I can't remember back in my day."
For posterity, Eaton's homer was the first walk-off shot to land in the pool and the 44th overall since the building opened in downtown Phoenix in 1998.
Eaton, asked about his first homer of the season and first walk-off shot of his career, said, "I blacked out. I don't know what happened."
"All I was thinking about was they needed me on-base," he added. "I just wanted to flick it into left, but I caught it out in front and I was happy with the result."
The crazy game had more twists and turns than a John Grisham novel. In the ninth, the Orioles loaded the bases with one out against D-backs closer Brad Zeigler, then tied it on a Nick Markakis sacrifice fly. Manny Machado then flied out, ending the threat. Will Nieves had given the D-backs a short-lived 6-5 lead when he led off the bottom of the eighth with his first homer of the season.
It even had a D-backs run allowed in the second when third-base umpire Alfonso Marquez ruled that Machado, the O's third baseman, interfered with baserunner Gerardo Parra as he made the turn before being thrown out handily at the plate.
"It was the wrong call, that's pretty obvious," Showalter said. "It's a lapse in judgment. He's out by three steps. Manny would have had to lay down in front of the bag and block his way. I watched it like you all have. It's just something that's unfortunate in a one-run game. One of our guys is not trying to mess it up. But he did."
Showalter's last game here managing the D-backs was on Oct. 1, 2000, an 11-4 loss to the Giants. His last win here was the previous night. There are always the games here on Tuesday and Wednesday to try to make up for that.
Since then Showalter's had stints managing the Rangers and Orioles, but he said the only event he attended in what was once called Bank One Ballpark was an alumni ceremony.
He's now leading an Orioles teams that, at 65-53, is hanging tough in the races for the American League East and the two AL Wild Cards slots.
On Monday night, the O's had leads of 2-0 and 4-2, but saw them evaporate, the latter when the D-backs tallied three times in the seventh off three Baltimore relievers to take a momentary 5-4 lead.
D-backs manager Kirk Gibson went to right-handed reliever J.J. Putz to open the eighth and after he recorded the first two outs, Gibson went for left-handed relief specialist Joe Thatcher to face Davis, the lefty-swinging power hitter.
Davis made the most of that decision, driving a full-count four-seam fastball the opposite way into the left-field bleachers to tie the score.
It would all be for naught in Showalter's big return, which in the end came down to O'Day's last pitch.
"Darren has been nails for us all year and he will be again," Showalter said. "I wish we could have done a better job in other places, to offset the things that didn't go our way with things that people see sometimes on the field that aren't there."