A World Engine is an enormous, tripod-type machine of Kryptonian design intended for use as a planet-altering device.
When paired with another World Engine and placed at opposite poles of a planet or planetoid, it generated a powerful beam that would run through the core of the world, altering density, gravity and atmosphere, making the world a habitable one for Kryptonians.
World Engines were deployed during Krypton's long-ago age of exploration in the hopes that other livable worlds could be found or altered to combat overpopulation and depletion of natural resources. Exploration was abandoned, however, in favor of remaining on Krypton and mining it's core for energy.
After being freed from the Phantom Zone, General Zod and his group of exiles, used their ship, the Black Zero, to travel to some of the worlds of the exploration, looking for a new home. They only found the remains of their fellow Kryptonians, who had been left to die when exploration was ceased. Scrounging what equipment they could, they came across a World Engine and harnessed it to the Black Zero and continued toward Earth to find Kal-El.
When Zod arrived at Earth and found Kal-El, he shared his plan to create a new Krypton on Earth, which would result in the genocide of humans while the planet would be violently altered to match Krypton's gravity, ecosystem and density. Zod had his scientists send the World Engine to the polar opposite end of Earth in the Indian Ocean where it landed and began to project it's beam through the planet's core. Above Metropolis, the Black Zero was "slaved" to the engine and the process of altering Earth began, resulting in the destruction of much of Metropolis and the deaths of much of its population.
The process was halted permanently when Superman flew to the site of the engine and destroyed it after a desperate battle with it's defenses.
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