Rob Gronkowski made sure to leave his mark on the Patriots before retiring from the NFL ... putting a HUGE dent in their 6th Lombardi trophy while trying to use it as a baseball bat.
Yes ... Gronk used a freakin' Super Bowl trophy to try to hit a baseball because ... we don't know why he would do this, actually.
It all went down when the Patriots were honored at Fenway Park before a Red Sox game earlier this month ... when players like Gronk and Julian Edelman were having some fun and *pretending* to hit baseballs while using the trophy as a bat.