History
Doctor Banner was supervising the trial of an experimental Gamma Bomb. After Ben Grimm, who was serving a security, managed to get Rick Jones out of testing range, they started up the process, but they suddenly saw that there was activity on the site. Once Ben saw what was going on, he took off to help.
Unfortunately, due to the damage the area sustained from the battle between Doctor Doom and Reed Richards, Banner couldn’t stop the explosion, causing Ben, after he managed to pull Reed into a protective trench, to transform into a hulkish creature.