The underground lake surrounded by the cavern. It contains special bioluminescent algae, making it the major light source for the entire D'ni city. The light dimmed and brightened on a regular, 30-hour cycle, marking days (a yahr) and nights.
Islands that dotted the Lake included Ae'gura, K'veer, Neref, Katha and a Cultural Center. Over 23 were owned by the government until King Emen sold them expensively to private citizens (mainly of the elite) who wished to build on them. He gave one of them to artistic Minor Guilds. Emen also opened a public ferry system aiding the access between the D'ni city, Ae'gura, and the surrounding smaller islands.
King Me'emen and Lanaren after him considered creating a tunnel that would connect the city proper with Ae'gura, and met heavy opposition by the Guilds. Lanaren abandoned the project, claiming that environmentalists were concerned that the lake level or even its algae would be affected. As this claim was never proven, the explanation was probably pretentious.
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.
When the cavern was first re-discovered, the algae were dormant. The DRC attempted to revitalize them, and managed to raise the light level slightly but remains constant. Their purpose is to return them to full health and resume their original cycle.
With the proper food (a form of highly processed grain) the level of light these algae give off can be dramatically increased temporarily.