Hulk is so fond of reminding people that “Hulk is the strongest there is!” that it’s easy for many fans to forget that super strength isn’t the Hulk’s main power. While being able to add hundreds of pounds of gamma-powered muscle to his skinny frame does grant Dr. Bruce Banner limitless superhuman strength, Banner’s actual super power is his ability to shapeshift or mutate his body into different powerful forms.
So, what does this mean? Well, for starters, Banner can give different versions of the Hulk superpowers that have nothing to do with physical strength. Many Hulks have the ability to see ghosts and astral bodies like Doctor Strange’s, thanks to Banner’s fear that the spirit of his father might be looking for him. And then there’s the useful power Banner created when the Hulk started spending more time in the water…
Yes, although this power is more typically linked to aquatic superheroes like Namor, the Sub-Mariner, the Incredible Hulk can breathe underwater, enabling him to swim and fight deep beneath the ocean indefinitely. Given all the inconvenient places Banner can Hulk out, this is a useful ability to have if he’s ever in danger of drowning, but it didn’t seem to exist during Banner’s early Hulk transformations.
Back in those early days, Hulk might find himself fighting underwater, but his ability to stay under the sea for extended periods of time was attributed to his superhumanly powerful lungs, which could hold tremendous amounts of oxygen and sustain the Green Goliath for a very long time. Even if Hulk ended up exerting himself in an underwater battle, his lungs somehow managed to keep his body going. This power also came in handy when Spider-Man accidentally punched the Hulk so hard that he sent him into orbit around the planet (yes, really). Although the Hulk was in his gray (and arguably weaker) form, his lungs managed to hold enough oxygen to keep him from immediately suffocating in the vacuum of outer space.
Naturally, since there isn’t any oxygen in outer space, Hulk can’t breathe in space – but eventually Banner gave additional Hulks even more adaptive capabilities, to the point where some powerful versions of Hulk can adapt so well to the hostile conditions of a vacuum that he can effectively live in outer space for extended periods of time. Eventually, however, some writers began allowing Banner to give the Hulk additional abilities that negated his need to hold his breath altogether.
In The Incredible Hulk #77, writer Peter David created a scenario where the Hulk and Banner realized he had a mechanism to prevent lung collapse. Thanks to a gland that would develop in his body whenever the Hulk was underwater, the Hulk could keep oxygen in his lungs even in water while avoiding decompression. This surprised Banner as he realized all of those years the Hulk held his breath were unnecessary. Most recently, Banner has apparently given the Hulk the power of immortality. Issues of the appropriately-named, The Immortal Hulk show Banner and the Hulk getting shot in the head, blown up, or even completely dismembered – yet thanks to the new powers Banner decided to give the new Hulk, the now-immortal Hulk can come back from practically anything.
It’s important to note, however, that as a shapeshifter, Banner can also deny certain versions of the Hulk these special abilities. Thus, it’s possible that some Hulks can’t develop that special gland to survive underwater or heal from fatal injuries. Nevertheless, it’s a testament to the Hulk’s ability to shapeshift that, given the right circumstances, he can breathe underwater, function in outer space, and basically live through anything.