Bruce Banner stores the extra mass he uses to become the Hulk in an alternate dimension.

Originally, Banner became a gray-skinned monster every night, but he got trapped in a space shuttle that exposed him to cosmic radiation which caused him to go green. So there are different versions of the Hulk that are tied to different split personalities of Banner stemming from his abusive childhood.

Hulk is capable of holding his breath for hours and has the ability to regenerate body parts.