Earth's core
Earth's core is a turmoil of superheated rock known as magma. It is potentially a source of immense power, provided it can be safely tapped. Accessing Earth's core can be a tricky prospect, however should you try, be very careful what you're getting into, as Earth's core might actually be Unicron, and he tends to get cranky if you drill into him.
Fiction
Generation 1 cartoon continuity
The Transformers cartoon
A shaft of crystal in the Arctic led all the way to Earth's core. The Decepticons tapped it to turn Earth's heat into energon, causing temperatures worldwide to plummet. After a battle with the Autobots resulted in the Decepticons retreating, the crystal was buried. Fire in the Sky Another similar crystal shaft was located in Peru. The Decepticons used the energies from the Earth's core to power a laser cannon until the Autobots again foiled their plans. Fire on the Mountain Never ones to give up, the Decepticons constructed a massive drill in a cave and began drilling their way to the Earth's core. This plan was undone when an Autobot attempt to gain control of Devastator resulted in the giant damaging the drill, endangering Earth while the Decepticons were still on it. Devastator successfully stopped the drill before Earth was destroyed. The Core
Japanese cartoon continuity
The Headmasters cartoon
The Decepticons constructed Death Towers as part of a plan to destroy the planet, drawing power from Earth's core and turning it into electromagnetic radiation. The Final Showdown on Earth (Part 1)
Marvel The Transformers comics
After traveling back in time, Galvatron set up a power siphon atop Mount Verona with the aim of harnessing the power of Earth's core, believing it would give him godlike power. Burning Sky! The siphon was eventually destroyed during a battle between Galvatron and the Autobots Ultra Magnus and Goldbug. Vicious Circle!
The Official Transformers: Generation 2 Annual
The Decepticons constructed huge rocket engines on Clarke Atoll, powering them with plasma from Earth's core in order to turn the planet into a huge spacecraft. The Autobots flew right into the engines and them engines up, because that sort of thing isn't cool. Moving Day Planet Earth
2001 Robots in Disguise cartoon
Optimus Prime lured Megatron into Earth's core via the Global Space Bridge for a final climactic battle and defeated the Decepticon leader with the Matrix Blade. The Final Battle
Titan Magazines Transformers movie comics
Following an unsuccessful attempt by Megatron to use the AllSpark to reformat Earth into a new Cybertron, Twilight's Last Gleaming, Part: 1 Optimus Prime worked out that the corruption had reached the Earth's core and infected it. Return to Cybertron: Part 1 The Autobots later successfully obtained a supply of Nucleon with which to save the planet. Return to Cybertron: Part 4
Aligned continuity family
Prime cartoon
Earth formed around Unicron, with the evil god becoming its core. One Shall Rise, Part 1 As a planetary alignment approached, Dark Energon began welling up from Unicron. One Shall Fall As Unicron began to wake One Shall Rise, Part 1 Team Prime took the desperate step of teaming up with Megatron and GroundBridging to the core. One Shall Rise, Part 2 Braving Unicron's antibodies, they fought their way to Unicron's spark chamber and Optimus Prime used the Matrix of Leadership to defeat the dark god. One Shall Rise, Part 3
2015 Robots in Disguise cartoon
Attempting to exact revenge against both Earth and Cybertron for their role in his perceived fall, Megatronus used a spark fuser to summon both the AllSpark and the remaining Anti-Spark of Unicron. The Anti-Spark consequently rose from Earth's core to Earth's surface, but after Optimus Prime destroyed the fuser, the Anti-Spark flew back down through the hole it had come from. Battlegrounds, Part 2
