Secret world seeds - Official Terraria Wiki

Secret world seeds are world seeds which have been discovered as secret easter eggs that generate worlds with strange or unique features. Unlike regular world seeds – which only affect the usual world structure – secret seeds can produce worlds with characteristics that would not be attainable through normal world generation (i.e., they can affect the game's mechanics while playing in that world). For instance, they may change enemy statistics and behavior, item drops, graphics, theme music, and more.

Approachable Game Design by AlodoTrobell on DeviantArt

Secret World Seeds - The Official Terraria Wiki, PDF, Wiki

The Constant - Official Terraria Wiki

The Constant - Terraria Wiki

Working as Designed - World Generation for Secret Seeds

Secret world seeds - Official Terraria Wiki

Secret world seeds - Official Terraria Wiki

Every NEW SECRET Seed in Terraria 1.4.4 Labor of Love

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Working as Designed - World Generation for Secret Seeds