Spawns a trapdoor that leads to the next floor upon activation.
There is a 10% chance the trapdoor will instead lead into a Crawl Space.
Some tiles will appear with a special graphic (for example grass or a piece of garbage in Basement). Using this item on these tiles will guarantee a Crawl Space. Only one trapdoor to the Crawl Space can be made per floor, as all subsequent uses will guarantee a trapdoor to the next floor.
Using the item on the 3 tiles directly horizontal to a door, including Secret Room entrances, will cause the trapdoor to spawn in a random spot next to Isaac instead, making this item an indirect aid to locating Secret Rooms.
If a special graphic is in these spots, the trapdoor will likely be moved to a spot that won't spawn a Crawl Space.
Spawning enough trapdoors can cause a trapdoor to spawn in these spots.
This item can be used to destroy rocks if used while Isaac is flying over a rock.
When used on the last floor of a challenge, a trapdoor to the next floor can spawn. If it is entered, Isaac must beat the boss of that floor to spawn the ending trophy.
Trapdoors will not spawn on challenges on floors at the final destination, however certain challenges like Red Redemption may still spawn a trapdoor leading to the next floor.
Synergies
Book of Virtues: Spawns a skull wisp on the middle ring that shoots bone tears.
Lil Portal: upon entering a trap door (not crawlspace), inside the lil portal special room, Isaac will be teleported to the I AM ERROR Room.
Interactions
Car Battery: In most cases, no extra effect. When used near the entrances to a room, the item can occasionally open 2 trapdoors at once, increasing the chance of opening a Crawl Space.
The name of this item is a reference to the movie Inception.
Gallery
3DS The icon of We Need To Go Deeper! as seen on the lower 3DS screen.
Bugs
Bug!
If a trapdoor is spawned inside of a Black Market with this item and is entered, the player is trapped in the Crawl Space unless they go to the next floor.
There is a 10% chance the trapdoor will instead lead into a 
Crawl Space.
Some tiles will appear with a special graphic (for example grass or a piece of garbage in 
Basement). Using this item on these tiles will guarantee a 
The item may be used on the Dirt Patches in the 
Dark Room to dig up a random type of chest.
Depths II to access 
Womb even before defeating 
Mom and gaining the achievement which 'unlocks' the Womb.
Utero II / 
Scarred Womb II, the trapdoor will always lead to 
Sheol.
Cathedral, 
Chest, 
??? and 
The Void (no trapdoors of any type will be generated).
Corpse II, 
Dad's Note floor, 
Home, and during the Ascent.
Secret Room entrances, will cause the trapdoor to spawn in a random spot next to Isaac instead, making this item an indirect aid to locating Secret Rooms.
Book of Virtues: Spawns a skull wisp on the middle ring that shoots bone tears.
Lil Portal: upon entering a trap door (not crawlspace), inside the lil portal special room, Isaac will be teleported to the 
I AM ERROR Room.
Car Battery: In most cases, no extra effect. When used near the entrances to a room, the item can occasionally open 2 trapdoors at once, 
3DS The icon of We Need To Go Deeper! as seen on the lower 3DS screen.
Bug!
If a trapdoor is spawned inside of a 
Black Market with this item and is entered, the player is trapped in the 
The Binding of Isaac: Rebirth
