This mental illness only manifests itself in folk who are born and grow up on the surface of Galactus. It comes about when these surface folk travel down into the bowels of the planet to the Deep, where sunlight cannot reach them.
Deep gloom is a relatively recent discovery, as only in the past few centuries have any surface dwellers begun to descend into the world of Deep Folk and stay there for any duration of time. This gloom manifests itself when surface folk spend too long away from sunlight and at such depths. It has never occurred in Deep Folk, however, as they are accustomed to the damp and the dark of the Deep world. As such, Deep Folk view this condition with contempt. The cultural differences between the surface and Deep dwellers are vast and many Deep Folk see surface dwellers as delicate and weak things, not built for life deep underground.
As its name suggests, the main symptom of Deep gloom is a heavy and pervasive gloom, weighing down on the mind and body. Some begin to feel claustrophobic in the stone tunnels and caverns, even those races such as dwarves which are more accustomed to subterranean lifestyle. Additionally, those more heavily affected can hear whispers or see hallucinations in the corners of their visions, and feel as though they are suffocating even when the air is completely fine.
Surface folk who make the decision to venture into the Deep are often given advice on how to counteract the effects of the Deep gloom, and are well aware of how it will affect their minds. Some of these methods are more effective than others. Magic users will cast spells that produce magical sunlight, which seems to be the most effective remedy thus far, despite its benefits being short-term. The best way to prevent Deep gloom is to stay well away from the world of the Deep.