Unclear why Mike hit the sand ... but it's all obviously part of a larger production -- hence the video cameras and crew members (all of whom were wearing masks btw, so props on that!).

In addition to the cage flip, Tyson was also spotted carrying buckets of bloody fish, tossing life rafts and wearing snorkeling equipment.

It almost seems like a promo for Shark Week.

Tyson just turned 54 years old this week ... and has said he wants to get back in the ring for some exhibition boxing matches -- but so far, nothing's a done deal.

The boxing legend told us he'd even be down to take on his old rival Evander Holyfield (who's also training for a comeback) but only if they could raise a bunch of money for charity.

"That would be awesome for charity," Tyson told us back in May ... "Can you imagine me and him going in the ring together?"

Tyson is also the focus of an upcoming movie -- with Jamie Foxx playing the boxer ... and he's already transforming his body into fighting shape.