Planetary core

A planetary core was the innermost portion of a planet.

Earth's core
When Earth formed billions of years ago, the Secret Heart became its core. (TV: The Runaway Bride)

Around 40,000 BC, the Euterpians planned to use Earth's core to refuel their mothership, but it was destroyed by solar flares before they could do so. (PROSE: Invasion of the Cat-People)

In 1984, the Dominators attempted to acquire Earth's core to fuel their fleet, but were defeated by UNIT. (COMIC: The Fires Down Below)

In 2007, the Torchwood Institute drilled a borehole to Earth's core under the Thames Flood Barrier in an attempt to free the Secret Heart. (TV: The Runaway Bride)

During their invasion of Earth, the Daleks attempted to remove the core and replace it with a power system that would allow them to pilot the planet anywhere in the universe, but they were defeated by the First Doctor. (TV: The Dalek Invasion of Earth) Iris Wildthyme visited the Earth's core. While there, she encountered reptile people and a "dashing old gentleman". (PROSE: From Wildthyme with Love)

Cores of other planets
Earth's was not the only planetary core the Daleks wanted. They mined the core of Arkheon in an attempt to acquire the Arkheon Threshold, (PROSE: Prisoner of the Daleks) mined the core of Galacton to fuel the Dalek Seventh Fleet (PROSE: Mission to Galacton) and drilled to the core of Keska in an attempt to destroy Gallifrey. (AUDIO: The Heart of the Battle)

The Kastrians' power source was drawn from the core of the planet Kastria. This was one of Eldrad's gifts to his people. (TV: The Hand of Fear)

The core of Lucifer was made of heavy elements which were the remains of the native Angels. (PROSE: Lucifer Rising)

The core of Spiridon was an allotrope of ice. (TV: Planet of the Daleks)

Among the exhibits in the Pentaquoteque Gallery were paintings made from metal from a giant gas planet's core and held in stasis. (COMIC: The Arts in Space)