The Theta underground city is the an underground complex near the base of Mount Hekla on Arcta, where the Thetas reside as the surface of the planet is mostly uninhabitable.
The city consists of hollowed out caverns, with a large central area where Thetas congregate or pass through. There are also a network of corridors and room areas, some containing hidden rooms where the Thetas secret their children, as Thetas are created to be sterile per the Cylons (TOS: "The Gun on Ice Planet Zero, Part II").