This underground chamber is a massive cylinder, 500 meters deep and 3 kilometers in diameter. The bottom 200 meters of the chamber is filled by the lake of light, the top 200 is occupied by the upside-down forest, and 100 meters of space separates them.
The lake of light is filled with water that glows, bright enough that the trees of the upside-down forest can live on the light. Many small fish and large crustaceans live in the lake, all of which are transluscent, the better to hide in the bright water. They live off of whatever falls from the forest.
The upsidedown forest is dominated by massive ironwood trees, on which grow numerous types of vines. The trees grow rocknuts and the vines grow berries. Ironhide monkeys swing from branch to branch, throwing rocknuts at intruders to knock them into the lake. A species of grey bee with cement hives pollinates the flowers of the trees and vines. Small birds live off of the berries, larger birds snatch fish from the lake below, and large birds prey on the monkey, as well as the occasional explorer.