The humps on Camerupt's back are formed by a transformation of its bones. They sometimes blast out molten magma. This Pokémon apparently erupts often when it is enraged.
Camerupt has a volcano inside its body. Magma of 18,000 degrees Fahrenheit courses through its body. Occasionally, the humps on this Pokémon's back erupt, spewing the superheated magma.