"What is this thing?"
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Reactor Core may refer to several rooms in the Metroid series:
Reactor Core (Frigate Orpheon)
Reactor Core (Sanctuary Fortress)
Central Reactor Core
Reactor Core Entrance
Additionally, the Metroid 1.5 document depicts a Reactor Core as being "the final area of the game where Samus sets the ship to self-destruct by overloading the engines."