Ultralisk cavern

The zerg ultralisk cavern is a shelled structure in which there are numerous high-radiation deposits and other dangerous substances. It is this extremely hostile environment which gives the ultralisk its toughness. In addition, it provides the genetic coding for the larva to morph into the breed itself.

Brood War
During the Brood War, the ultralisk caverns could also provide additional genetic mutations for Anabolic Synthesis (for faster movement) and Chitinous Plating (for improved armor).

StarCraft II
In StarCraft II the ultralisk cavern is still required for production of ultralisks.