The Nacenes having accidentally removed all of the nucleogenic particles from the atmosphere, the planet slowly began turning into a desert. The local Ocampa were not technologically advanced enough to cope with the changes to their planet, and, experiencing guilt, the Nacenes created the city so as to preserve the Ocampa civilization.
Simply referred to as "the City," the underground settlement had ten levels as well as vast open spaces that allowed its inhabitants to look out over the levels above or below them. The lowest tier of the city housed its medical, life support and power generation systems. While food synthesizers existed throughout the city, the Ocampa had their own hydroponic gardens where they could grown some of their own crops. Each level of the city had at least one hydroponic garden.
Although many Ocampa believed the city to be sealed off from the surface, fissures in its walls led all the way up to ground level. The Caretaker actively discouraged the Ocampa from exploring the fissures and utilized his power to close up any which he discovered.
In 2371 the Caretaker transported B'Elanna Torres and Ensign Harry Kim down into the Ocampa city, claiming they had contracted a virus and requesting the local Ocampa to care for them. In reality, the Caretaker was dying and was seeking species who were a genetic match to take over and care for the Ocampa as he had done. The kidnapping of B'Elanna and Kim brought Captain Kathryn Janeway and members of her crew down into the city to rescue them.
Following the Caretaker's death and the USS Voyager's departure several days later, the Ocampa city was left with a five year energy supply that would run out in 2376. Routine maintenance of the city continued, although different sections of the city began to develop their own governing councils, a feature lacking previously. Some Ocampa, fearing the collapse of their way of life, began to research and improve their scientific knowledge. These Ocampa were eventually able to develop crude forms of power and life support generation for the city. Other Ocampa, usually younger in age, left and migrated to the surface. (VOY episode & novelization: Caretaker, Decipher RPG module: Worlds)