Big Booty

Biome Chests sit locked in the dungeon - Jungle, Corruption, Crimson, Hallowed, Frozen (and Desert) - each wanting its matching Biome Key, an exceedingly rare (1-in-2500) drop from enemies in that biome, usable only after Plantera. Farm your chosen biome with battle potions and a water candle; the Jungle Key's Piranha Gun and the Hallowed Key's Rainbow Gun (Rainbows and Unicorns) are popular targets. Unlock any one chest and it pops.