The Fiilestii module was a large, floating artificial habitat, one of hundreds of biomes making up the Web of Worlds in the First Federation. It was located within the atmosphere of the class J gas giant Cherela.
The Fiilestii module recreated a portion of the avianoid Fiilestii's homeworld, which had been destroyed by Dassik. It was more than 1,000 years old, 1000-kilometers wide, and supported a population of more than one million Filestii. Kilometers-high arcology towers shaped like narrow cones rose from the ground, with several disc-shaped levels embedded along it. Each disc measured dozens of meters thick, hundreds of meters in radius, and contained residences and job sites for the Fiilestii.
In 2269, Pavel Chekov and Hikaru Sulu explored the Fiilestii module in wingsuits. They were present when gravimetric fluctuations began to destabilize its antigravity system. Emergency evacuations began, but the some of the arcology towers began to collapse and people fell to their deaths. James T. Kirk offered to assist, and the USS Enterprise itself was beamed closer to the module. While the ship's tractor beams shored up the failing antigravs and its transporters worked overtime to beam out survivors, the efforts were not enough, and the module eventually collapsed helplessly downward into Cherela's atmosphere with several thousand inhabitants still inside. Kirk was arrested as a scapegoat, with First Federation officials blaming the ship's tractor beams as interfering with their technology and causing the collapse. (TOS novel: The Face of the Unknown)