Here's footage of former Heisman Trophy winner Johnny Manziel leaping off a rooftop and doing a front flip into a pool ... and it's INSANE!!!

Unclear where the debauchery went down ... but Manziel posted the video Sunday -- showing a slow-mo shot of him soaring roughly 20 feet through the air before splashing down.

It's pretty scary ... the pool ain't exactly huge, and he appears to be just feet away from the shallow end!!!

Of course, jumping off high things into water is becoming old hat for the recently retired football player ... it was just a few weeks ago when he posted vid of him throwing himself off a cliff.

Don't worry, John's still alive and well ... and hey, the video is pretty cool.