The City formed the core of the D'ni empire, and most of the D'ni structures and population was centered here. The main Cavern which houses the D'ni city holds an underground lake which contains an unidentified species of luminescent algae. These algae gave off enough light to light up the entire Cavern, in a 30-hour cycle which formed the natural day and night for the D'ni. Through unknown means of poisoning the algae nearly all died off, and still have not fully recovered. The result is that level of light these algae give off was diminished. It has, however, been revealed in Uru: The Path of the Shell that with the proper food (a form of highly processed grain) the level of light these algae give off can be dramatically increased temporarily.