Biome Keys

When in hardmode every enemy has a 0.025% (1 in 4000) chance of dropping a Dungeon Key Mold. The type of Dungeon Key Mold that is dropped depends on the biome you are currently in (Jungle, Corruption, Crimson, Hallow or Ice). These molds are then crafted into keys by using a Temple Key (dropped by Plantera) and Hardmode "Mechanical" boss Souls. These keys can then be used on their respective Dungeon Chest.

Obtaining Key Molds
The Dungeon Key Molds can be obtained relatively easily following this guide. Guide:Easy way to get Keys