Planetary core

A planetary core or simply core is the innermost layer at the center of a planet. In the same way that a stellar core was the center of a star.

In 2258 of an alternate reality, Nero used a mining drill on the Narada to dig to the core of Vulcan. He then put red matter into the core of Vulcan creating a black hole and destroying the planet. 

The planet Terra 10 had a molten core surrounded by a mantle and crust that was entirely crystalline. 

The moon Jeraddo around Bajor had a molten core, and in 2369, there were plans to transfer energy from this core, forcing the evacuation of the residents.