Bizarro World

An exact and opposite mirroring of our world, Bizarro World is "an upside down" version of reality. In terms of string physics, it is the closest differing frequency past the natural differing in frequency across the physical dimension of space-- it is closer by order of magnitude than alternate realities or dimensions, but farther than universes. 
The first known encounter between Bizarro World and the "surface world" occured when Lex Luthor performed an early trans-realital experiment, in which he attempted to access a subservient Superman from another reality that he could use as his pawn. However, being the first on the bleeding edge of inter-reality travel, with all of Luthor's genius not even he could even realize that his acceleration of the strings was off by some orders of magnitude. When Luthor performed the experiment, he inadvertently pulled the Bizarro World Superman, Tnek Kralc, through to his world (where is Superman story). Almost instantly rejecting Bizarro Superman as a failure, Bizarro would escape confinement and unintentionally cause destruction in Metropolis. The Superman of the "Rightside Up" world would arrive to stop him, finding that their power sets were equal and opposite-- Superman had frost breath and heat vision, and Bizarro had heat breath and frost vision. After their initial fight, Superman went to Lex Luthor to attempt to help Bizarro return to his world. Instead, Luthor saw this as an opportunity to have a test subject to learn Superman's powers first hand so he could find methods of countering them. LOD.
Later, in an act of kindness, Jason Todd would break into LexCorp and attempt to return Bizarro to his homeworld, only for the plan to backfire-- instead of sending Bizarro Superman back, the rest of the Bizarro Justice League arrived through to the Rightside Up.