The following section contains information from a previous version of StarCraft II which is no longer valid.
The ultralisk cavern was one of the last zerg buildings to be overhauled as StarCraft II neared completion. Many different attempts were made to figure out the rhyme and reason for the zerg building designs. Several buildings were visually interesting but had no correlation to the Zerg units they enabled. There had been at least three different versions of the ultralisk cavern before the final artwork was created.
The final zerg buildings we produced had many visual elements in common with the units they unlocked. The main idea behind the ultralisk cavern was to display the distinct ultralisk features on a revamped version the building from the first game.
At one point the cavern was represented by what would become the roach warren.
This ability allows ultralisks to quickly (speed 3.75) tunnel underground and charge forward to resurface in the midst of battle. This is to combat ultralisks being unable to get into battle due to their size or other units being in their way. Vision to the attacked area is not necessary.
It seems to deal damage when the ultralisk emerges from the ground.