The inner Sun of Karda Nui

The saltwater that comes into the heart creates a giant waterfall dropping out of the dome and as it hits the surface of silver protodermis strong light is emitted creating an inner sun. This is the light source of Karda Nui.

On this early concept drawing you can see the big fall, the inner sun and the fires of the Av-Matoran trying to survive on the ledges of the stalactites that are impaled in the swamp.