The Doomsday Thermal Reactor was an Atlantean weapon of mass destruction.
Aquaman built the Doomsday Thermal Reactor as a last line of defense should the surface world attack. Positioned at the North Pole, it could melt the polar ice cap and flood a large part of the planet. However, Aquaman never armed it, and it lay unused for a long time.
With plutonium stolen from the USS Defiant, Orm armed it, and set it off as part of his usurpation of the throne. Aquaman challenged him, but in the fight, Orm deliberately destroyed the control panel. Batman and Green Lantern managed to shut the machine down by removing the plutonium.