When Ghosts have fulfilled what keeps them on earth, a Crossover Door or "Door to Death" immediately appears in their immediate vicinity. In appearance, the door is entirely ordinary, but leads to the next stage of their existence (or possibly their extinction). This is their "death."
When George asks Annie what is on the other side of the door ("Is it something good or something else?") she replies "it's probably something else." It is sometimes referred to as a good thing because it is seen as an end, as stated by Mitchell in Episode 1.1. Annie then asks George not to look inside the door once she opens it. However, when Gilbert finally dies in Episode 1.3, the open door is shown to the viewer as leading to a brightly illuminated place (much like the common interpretation of Heaven) and he smiles when he looks inside. This version of the afterlife is shared by the ghost Jimmy in Series 2, who enters a door of shining light gladly. In Episode 2.2, the inside of the door is shown as a sinister red corridor, possibly representing Hell - when Saul steps inside, the door slams shut and he instantly fades from view. In Episode 2.6 death's door appears again, but shining from behind it is a bright violet light.
Death's door appeared in the second series finale, opening to reveal a black void, which managed to drag both Annie and a newly-made ghost inside. Annie later became the first person shown to temporarily return from behind the door, by appearing as a blue faint spirit with the ability to physically interact with and appear to humans. When Kemp was pushed through the door by Annie, he disappeared. Annie mentions that the Gatekeepers are angry about this, as they "don't have a form for it."
This feat was repeated in Becoming Human, when Adam, Christa and Matt, having discovered that Matt had been murdered by his teacher Mr Rowe, shoved Mr Rowe through Matt's door when he tried to attack Adam and Christa, although Adam speculated that this would result in problems in the future.
Mitchell was able to enter another man's Door to search for Annie. Passing through a form of Purgatory where he witnessed some of his past crimes, Mitchell was able to find Annie, but it is unclear if this is something that others would experience as Mitchell's efforts were interfered with by Lia Shaman, one of Mitchell's victims from the Box Tunnel 20 Massacre. It was later revealed that his efforts caused a disruption of the usual balance of life and death, resulting in the creation of around five 'zombies' as Mitchell's use of the door prevented their spirits from passing on until their bodies sustained enough damage or decay to stop them continuing.