WARNING: This is pretty gnarly. And bloody. 

UFC star Paige VanZant tore a massive bloody hole in her foot -- and the photo is pretty graphic. 

23-year-old VanZant didn't say how it happened ... but with rumors she could return to the Octagon at UFC 216, it's likely she opened the gash during a training session. 

Pretty gross ... but this kinda stuff happens. 

UFC 216 is scheduled for Oct. 7 -- and reports say PVZ will likely fight Jessica Eye

If her foot heals ...