Basement

The Basement is one of the environments in Chapter 1. Unlocked at the beginning of the game, it is considered the easiest of the environments in the game. The theme of the Basement is flies and poop.

The Basement's harder unlockable variants are and the.

After is unlocked, a locked door similar to the one leading to a Treasure Room will appear in this floor's Boss Room upon defeating the boss leading to  /. Like a Treasure Room, this door requires a key to be unlocked. Unlike Angel Rooms and Devil Rooms, this door never disappears.

Monsters

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Music
The music for the Basement is titled Diptera Sonata. Diptera is the order of true flies. Thus, Diptera Sonata literally means "Fly Sonata".

Binding of Isaac: Rebirth OST