Despite being one of the strongest heroes in the Marvel Universe, Bruce Banner and his alternate personality, the Hulk, have been killed quite a few times. Currently, Al Ewing and Joe Bennett have explored the many deaths of Bruce Banner and the Hulk with their ongoing series The Immortal Hulk. This effectively canonized the idea that while Bruce Banner can die, the Hulk is immortal. But how did the Hulk reach this terrifying new peak of power?
After Bruce used the Hulk to absorb radiation from an experimental reactor in Greg Pak and Frank Cho's Totally Awesome Hulk #1, the boy genius Amadeus Cho used special nanites to transfer Banner's powers to himself in Pak and Cho's Totally Awesome Hulk #4, preventing the Hulk from detonating and saving Banner's life in the process. As a result of this power transfer Banner was no longer able to become the Hulk, leading him to experiment on himself with gamma-irradiated cells while in seclusion at his Utah lab.
Around this time, Ulysses, a new Inhuman with the power of precognition, foretold that Bruce would once again transform into the Hulk and begin a deadly onslaught. A large group of heroes, led by Captain Marvel and Iron Man, confronted Bruce outside his lab. After the heroes learned of Bruce's experiments he became agitated, leading Hawkeye to shoot him in the head with a special arrow provided earlier by Banner himself.
Despite his death at the hands of his fellow Avenger, Banner's Hulk would appear quite frequently over the next few years. His first appearance following his death came in Gerry Duggan and Ryan Stegman's Uncanny Avengers #14 and #15 which saw the Hand resurrect and implement a mindless Hulk before he was stopped by Brother Voodoo. Bruce and the Hulk were once again resurrected in Nick Spencer, Joshua Cassara, Rod Reis, and Andrea Sorrentino's Secret Empire #5, this time by Steve Rogers' evil doppelganger and the Hydra scientist Arnim Zola. While Bruce refused to help fight his friends, his Hulk side complied and helped destroy the resistance base known as the Mount, under which he was subsequently buried.
The Hulk's final resurrection came during the "No Surrender" arc at the hands of an elder of the universe known as the Challenger in Al Ewing, Mark Waid, Jim Zub, and Kim Jacinto's Avengers #679. The Hulk initially agreed to help the Challenger due to his anger at humanity for not letting him stay dead, but he would later turn on the elder when Wonder Man convinced him that the Challenger was at fault for bringing him back. It was at this point that the Hulk realized that the succession of resurrections was not a coincidence but instead a manifestation of his innate immortality.
At this point, Ewing and Bennet's Immortal Hulk series begins, with Bruce keeping under the radar and traveling the country during the day as the night was Hulk's time. With the Hulk in charge, the connection between immortality, gamma radiation, the green door, and the One Below All have all been explored. While the Hulk is currently having a rough go of it with the death of devil hulk and the disappearance of Bruce Banner, his innate immortality makes it unlikely that he'll be kept down for long.