Planetary core

The planetary core is the inner most layers at the center of a planet. Cores can be composed of solid or liquid layers. (ST reference: Star Charts)

In 2267 Commander Krell reassured his men they would look back on the sedate world of Neural with pride from palaces once they had conquered it and mined it's rich mineral wealth to the core. (TOS comic: Klingons: Blood Will Tell #3)

One of the Dax symbiont's hosts was involved in operations working with Hortas to mine planetary cores. (DS9 novel: Devil in the Sky)