This article explores the appearance, in Jonathan Lethem's Chronic City and Ben Lerner's 10:04, of ontological instability and disturbing nonhuman presences in urban space. Developing the work of Brian McHale in Postmodernist Fiction, ontological instability is understood here as shifts in what is deemed as real or unreal in the storyworld, and the resulting uncertainty (for characters as well as for readers) as to the stable ontological attributes of the narrated world. Such shifts feed into broader apocalyptic undercurrents in Lethem's and Lerner's novels. They are also important for an understanding of how early twenty-first-century fictional texts come to grips with complex environmental threats, and have relevance for the relationship between human vision, consciousness, and the environment. In Chronic City and 10:04, I will argue, a sense of unsettling nonhuman presence is realized in continuous references to menacing weather conditions and occurrences that threaten the ontological stability of the narrated storyworld.