Mountains of Madness (Constantinople) is a Lovecraftian location in the Byzantine arc of Uchronic Paris, where the group was trapped without knowing it during a fungal mindscape. When they broke the illusion, they discovered they had never left. The name directly evokes H. P. Lovecraft’s At the Mountains of Madness.
The mountains are adjacent to, or part of, the Valley of Death. The fungal creature that generates the mindscape lives in this zone.