The Core Muncher is an animal that burrows through a planet's crust. Zim explains to his intern "Derb" (actually Dib in disguise), that he got the Core Muncher at a Bazoogian Bio-Swap and Crafts fair. He further states that he's hollowed out its brain and implanted a control bubble, and that he intends to use it to create new volcanos all over the Earth.
It's at this point that Dib reveals himself to a stunned Zim. He then shocks the Core Muncher with a power cable. This causes the Core Muncher, which Zim apparently named "Linda", to go berserk, and it burrows through the floor of Zim's base, carrying him away with it, while filling the base with lava.
It is unknown if the Core Muncher survived this experience.