The threat of these hostile mutants, in addition to exposure to the Fog, drove many human inhabitants of the Island insane, turning them into psychotic cannibals known as trappers. These creatures are highly aggressive, and sometimes attack Far Harbor. The native fauna of the Island was mutated by constant exposure to the Fog, creating abominations such as anglers, gulpers and fog crawlers. However, Old Longfellow recounts that in the days of his early childhood, the whole island was covered in the Fog, and that many people on the island are quick to forget its threats in the years it recedes, as they have grown complacent with the relative prosperity that followed. The residents of Far Harbor believed that the Children of Atom were somehow responsible for spreading the Fog, while the Children believe that its growth was Atom's reward for their faith.

However, ever since the arrival of the Children of Atom, it began steadily growing until it threatened to cover the entire island. Originally, the Fog was merely an occasional presence. Condensed fog can then be used to craft several useful items. While the Fog has engulfed almost the entire island by 2287, fog condensers created by Acadia are capable of protecting small areas from the effects of the Fog by condensing the Fog into a liquid form. The Fog is radioactive in nature, making it a hazard to the human residents of the Island, except for certain members of the Children of Atom and the synths of Acadia, who are immune to radiation. It is celebrated as a blessing of Atom by the Church of the Children of Atom and feared by the people of Far Harbor. The Fog is a hazardous weather condition present on the Island in 2287.

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