The Green Door is a metaphysical barrier that connects directly to the Below-Place, a dimension that was part of the Multiverse. After death, the souls of gamma mutates cross into the Below-Place through the Green Door and are resurrected. They are then capable of returning to Earth while the Green Door remains open, but would possess no memory.
History[]
Discovered by Brian Banner[]
While studying Gamma Radiation from a secret log that was stored in one of Howard Stark's files during his time in S.H.I.E.L.D., Doctor Brian David Banner learned about the existence of the Green Door. Upon finding it, he walked over to touch it, but would suddenly catch a glimpse of the One Below All, a grotesque presence in the Below-Place. Believing that this Green Door was the way into a different dimension, his obsession would lead to his death when he died from radiation poisoning. However, he would then come back to life as a gamma mutate known as the "Devil Hulk", but didn't make it back to Earth when the Green Door closed and remained in the Below-Place.
Gamma Bomb Incident[]
After Brian's death, his son Bruce continued on with his father's research experimented with gamma radiation in hopes of making soldiers immune to radiation poisoning. Under the orders of General Thaddeus Elliot Ross, Bruce designed a Gamma Bomb for a strategic defense project. Once the test was ready, a young teenager named Richard Milhouse Jones ventured onto the test site, leading Doctor Banner to rush over and save him. Before Banner could get himself to safety, the Gamma Bomb detonated, and intense waves of radiation reached the surface. Banner was irradiated with highly charged radioactive particles.
The radiation caused Bruce to die, but the effects of the Gamma Bomb opened the Green Door and his soul entered the Below-Place, a dimension located within the Multiverse. After being resurrected and returning to Earth while the Green Door remained opened, Bruce had no memory during his time in the Below-Place, but would learn that his own body was altered, in which he frequently transform into the vastly powerful green-skinned humanoid monster whom General Ross referred him as "The Hulk".
External Links[]
Green Door on the Marvel Wiki