The Hulk is undoubtedly one of the most powerful characters in the Marvel Universe,
but just because he is powerful on a cosmic scale doesn't mean he is immune to any harm. In fact, Marvel Comics reveals the one thing that can truly kill him.
Bruce Banner, better known as the Hulk, was exposed to a deadly dosage of radiation, which caused him to mutate into the gamma mutant.
However, the gamma radiation permanently changed his DNA, transforming him into a beast capable of unmatched global annihilation instead of killing him.
Even though the Hulk has been a staple of Marvel Comics for years, it's possible that his full potential wasn't realised until Earth War Hulk,
in which the Illuminati sends the Hulk off world to wage war against Earth's heroes.