Skifander

The Lost City of Skifander is the homeland of Zeetha, daughter of Chump. It appears to be a matriarchal society with a strong warrior tradition, but most things about it are mysterious- so mysterious that hardly anyone has even heard of the place.

Characters with apparent first-hand knowledge of Skifander include Zeetha herself, Klaus Wulfenbach, Lucrezia Mongfish, and Barry Heterodyne. Agatha and the members of Master Payne's Circus of Adventure have all heard about it second-hand.

Fan Theories
Quite a bit more has been hinted at than actually spelled out in canon. For example, it is widely suspected that this is where Klaus disappeared to, and the birthplace of his son.