Jungle Chest

The chest is already sitting in the Dungeon from the moment the world is generated, locked, and no pickaxe or lockpick touches it. You need the Jungle Key, which only turns up in Hardmode from enemies killed inside the Jungle. Even with the key in hand the lock will not move until Plantera has been beaten once in that world. Before then the key carries a warning that something in the Jungle has cursed it.

A world places five of these, one for each biome it actually contains, and only ever one evil variant matching whichever evil it rolled. Drunk worlds are the exception and get both. Only the chests generated with the world hold anything, so a biome chest you obtain and place yourself comes up empty.

The reason to come here is the Piranha Gun. Each of the five chests holds exactly one weapon like this and each one belongs to a different class, which is why most players chase the single key that matches how they are playing instead of collecting the set. Everything else in the chest is ordinary filler picked by how deep in the Dungeon it generated, plus a chance at the Remnants of Devotion painting.

Key drops are rare enough that planning a run around one is a mistake. Build a farm in the Jungle and let it work in the background while you push progression elsewhere, or simply accept that the gun may show up on its own over a long Hardmode session. Going out specifically to grind for it usually costs more time than the weapon saves.