Polluted Cistern // Dim Oubliette

If an effect allows you to cast a spell with certain characteristics, consider only the characteristics of the door you're casting. For example, if an effect allows you to cast a permanent spell with mana value 3 or less from among cards in your graveyard, you could cast Bottomless Pool this way, but not Locker Room.

Any time you have priority during a main phase of your turn and the stack is empty, you may pay the mana cost of a locked door (also called its "unlock cost"). That door becomes unlocked. This is a special action. It doesn't use the stack and can't be responded to.

While in any zone other than the stack or the battlefield, a Room card's characteristics are a combination of its two doors. For example, Bottomless Pool // Locker Room has a mana value of 6 while it's in your library. If an effect allows you to search your library for a card with mana value 4 or less, you can't find Bottomless Pool // Locker Room.

To cast a Room spell, choose a half (or "door") to cast. There's no way to cast both halves of a Room card. When the Room spell resolves, the corresponding door becomes unlocked as the Room enters.

If neither door of a Room is unlocked, it's a Room enchantment with no name and no abilities.

While on the battlefield, a Room's characteristics are a combination of the characteristics of its unlocked doors. For example, if Bottomless Pool // Locker Room is on the battlefield with both doors unlocked, its names are Bottomless Pool and Locker Room, its mana value is 6, it's a Room Enchantment, and it has the abilities in each door's text box.