??? (Floor)

???, referred to as Blue Womb or Dead Womb in the OST, is a chapter added in The Binding of Isaac: Afterbirth. It is unlocked by defeating Mom's Heart for the 10th time.

It is accessed by defeating Mom's Heart or It Lives! within 30 minutes. Leaving and re-entering the Boss Room after the timer has reached 30 minutes will make the entrance to ??? disappear. If the player is fast enough, both Boss Rush and ??? can be completed in a single run. The boss of the floor is always Hush.

It contains the following:
 * 4 Locked Chests in the main room.
 * Unlike the ones in, these are just regular golden chests and don't guarantee an item drop.
 * 2 locked Treasure Rooms, both of them offering two choices of items as if was active.
 * A Shop that always contains 6 items, even if Shop Upgrade Lvl. 4 has not been unlocked.
 * A 2x2 boss room that contains Hush.
 * The door leading to the boss room might blend in with the northern wall but will slide away upon approaching it.

It is advised to only enter ??? with very powerful items and/or a set of good dodging skills because Hush ranks amongst the most difficult bosses of the entire game.

After Hush is defeated, one of two things will happen:
 * If this is the first time Hush has been defeated, a giant golden chest will appear in the middle of the room. Jumping into it ends the run and gives ending 17.
 * If Hush has been defeated before, a trapdoor and a beam of light leading to or the  will appear.
 * If Hush has been defeated before but Sheol and the Cathedral have not been unlocked, the giant golden chest will appear instead.
 * Additionally, an exit will spawn in the corner of the room, containing a Portal leading to.

Trivia

 * The displayed name of the chapter is ???, but the chapter is referred to as Blue Womb by the achievement, using the same logic as the character.
 * The character ??? and the chapter ??? have the same unlock requirements.
 * ??? is unique as the only chapter that can be optionally done between mandatory chapters.
 * It is mandatory, however, to complete this chapter to unlock.
 * It is also mandatory to complete three times to unlock, as well as the rest of the alternative chapters, the passage of which eventually gives access to.
 * ??? is the smallest chapter, only containing 6 normally accessible rooms.
 * In Isaac's last will, ??? is referred to as "some barren place".
 * This may be a reference to the fact that infertile women were historically referred to as "barren".
 * ???'s icon on the progress bar is actually a recolor of 's.
 * Similarly, the trapdoor leading to ??? is a recolor of the trapdoor leading to the first and second floor of Chapter 4.

Music
The ???'s music is called Mater Prope or Nativitate. These names are in Latin and translate to "Almost Mother" and "Birth" respectively.

The tracks that play during the battle against Hush are called Ascension (the ??? boss fight music; played during Hush's first phase, which is similar to the ??? boss) and Morituros (during the second phase).

Binding of Isaac: Afterbirth OST