User blog comment:FairyRaven/Fairy Tail Chapters 338, 339, 340: Review and Commentary/@comment-5598380-20130706211549/@comment-5598380-20130707043307

In One Piece, Skypiea is an island in the sky 10000 meters above the ground. Fishman Island is an island 10000 below sea level. Since Mashima is a fan of One Piece, maybe he based the story of the parallel world Edolas from the Skypiea Arc of One Piece.

Edolas is a parallel world above Earthland. So maybe there's also a parallel world below Earthland called the "Underworld", "Netherworld" or whatever. And maybe FT will travel deep down.