It's common knowledge that the Hulk is one of the most powerful beings in the Marvel Universe.
However, the fact that he can smash buildings doesn't make him impervious to harm.
Marvel Comics has revealed the one thing that can really kill him.
After being exposed to a potentially fatal dosage of radiation, Bruce Banner, called the Hulk, became the gamma mutant.
The gamma radiation did not kill him, but permanently mutated his DNA.
Making him a monster capable of unimaginable harm on a galactic scale.