Turns out, the foot injury that's been plaguing him during Oakland Raiders training camp stems from a "cryogenic chamber mishap" ... this according to NFL Network's Ian Rapoport.
31-year-old Brown, who signed a 3-year $50 MILLION deal with the Raiders, has been nursing some pretty gross-looking feet ... but he hasn't been clear on what caused the injury.
There's been rumblings that he went into a cryogenic chamber without proper foot protection -- but neither Brown nor his representatives have said anything on the record about it.
FYI, cryo chambers can get to about 200 degree below Fahrenheit -- and they're typically used for workout recovery.