A Beached Thing comes into existence when the deceased soul of a living being fails to cross through the Beach into the world of the dead, consequently becoming "stranded" in the world of the living, with their connection to the other side maintained through an umbilical cord. The Death Stranding occurred concurrently with the first recorded sighting of a BT by a doctor performing a Caesarean section on a brain dead mother. Due to the Death Stranding, when a human being dies, their "ka" (soul) is still connected to and will attempt to return to their "ha" (body) after death. The only way to prevent this is to destroy the body, usually through cremation at an incinerator; if this is not carried out, the ka will return to the ha and then become a BT, stranded in the world of the living, risking a Voidout.

BTs can be perceived by individuals suffering from DOOMS, or via a trance connection to bridge babies, which exist between the worlds of the living and dead. The extent to which DOOMS sufferers can interact with BTs varies; Sam is capable of sensing BTs, but cannot see them without the assistance of Lou. Fragile, meanwhile, is capable of seeing BTs without aid. With a DOOMS level of seven at minimum, Higgs is capable of not only seeing BTs, but also controlling them at will.

It is never clear if the Beached Things are all stranded human souls, native entities to the 'other side', or a mixture. It is safe to assume that Gazer-types are all the result of human deaths, but very little is explained regarding the nature or origins of the larger BTs. Higgs' Colossus, however, would seem to be an entity that is entirely of his own creation and under his control, suggesting that it is at least possible for BTs to arise from sources other than human death. It is likewise unclear if the BT's behaviour is predatory or not: The grappling and grasping of Gazer-type BTs could be interpreted as a desperate need for physical contact, or even a plea for help. Heartman at one point speculates that the human hand-prints that appear during close encounters could be interpreted as the BTs reaching out to us.