Onigafuchi Swamp (鬼ヶ淵沼・onigafuchi numa) is a swamp in Hinamizawa. "Onigafuchi" means "depths/deep pool" (淵) of (ヶas a possessive marker) demons (鬼). The village was originally known as "Onigafuchi," because of the legends of demons in Hinamizawa's past.
The swamp is considered dangerous, surrounded by many signs forbidding entry. In the Sound Novel of Tatarigoroshi-hen, Keiichi notes that some boys will take bait to catch the giant crawfish that live in the swamp. After killing Hōjō Teppei, he disposes of Teppei’s motorcycle and Satoshi's bat, which he used to kill Teppei, in the swamp to hide any evidence of the man’s murder. Rika's mother is reported to have committed suicide by drowning herself in it, likely to appease Oyashiro-sama’s anger when her husband died, though she was murdered by Takano Miyo through brain dissection in reality.
The claimed cause of the Great Hinamizawa Disaster was volcanic poisonous gas released from the swamp. The government drained it and filled it with concrete to prevent another eruption which, in reality, would not work as seen in many arcs.