The Antichthon was the name used by ancient philosophers, for example Cicero of Rome in his Somnium Scipionis, to describe the mythical southern continent or Antipodes, which many at that time believed to exist.
The existence of vast southern continents on the Earth had been hypothesized for many hundreds of years before Cicero, most recently by the ancient Greeks, who approached the idea from a scientific angle, stating that because the Earth was a sphere (yes, everyone, Columbus was not the first to come up with that idea), there must exist huge continents below the equator to counterbalance the weight of the northern continents or oikoumene, otherwise the Earth would fly apart.
In fact, it was this belief in the Antichthon which drove human exploration for so long, probably until the Americas were discovered again and it became clear exactly how huge they were. When Australia and Antarctica were actually discovered, it was probably something of an anticlimax after the enormous riches and complex societies which were available for the 'civilized' world to loot in North and South America. Kangaroos, koalas, and a stone age indigenous people. Mile-thick sheets of ice. Let's go back to the Chthon boys, it's better up there.