The planet nearest to the sun.
In February 1952, in a ruse designed to fool the highly secretive Crime Czar, Superman builds a robot called Krag—supposedly a super-powered alien from Mercury—and has Krag announce his intention to become the "King of Crime." After staging the defeat of Superman, Krag is invited to meet with the Crime Czar. Subsequently, Krag brings the Crime Czar to jail and Superman reveals his deception (Act No. 165: "The Man Who Conquered Superman!").