We're representing the fact that the Earth is actually hollow. The inner Earth (Agharta) accommodates a base of operations of an advanced alien race (Bartixians). You can see Agharta through the hole opened up for Bartixian spaceships, as the buggers tend to zoom in and out of it once in a while through the crust itself, instead of using the large polar holes.



In other words, it's a feature.