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Memory Beta, non-canon Star Trek Wiki

Cross section through a class M world, featuring the inner and outer cores

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

Hundreds of years prior to 2266, a meteorite collided with the planet Metamorpha and penetrated to its core, bringing about rapid chemical changes. The planet was stabilized when Spock had an atomic missile carrying certain chemicals fired into the core. (TOS comic: "The Peril of Planet Quick Change")

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 its rich mineral wealth to the core. (TOS comic: "The Order of Things")

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