Crawl Space

Crawl Spaces (also known as Retro Treasure Rooms) are small secret rooms that can be entered by touching trapdoors with ladders in them. They can be found in 8 different ways:


 * 1) Using the We Need to Go Deeper! activated item (10% chance)
 * 2) Using the Ehwaz rune (8% chance)
 * 3) Bombing a Clean Bedroom rug (100% chance)
 * 4) Destroying a rock (unknown very small chance)
 * 5)  Using Chaos Card on Great Gideon (100% chance)
 * 6)  Getting hit while having Cracked Orb (unknown chance)
 * 7)  Using  (100% chance)

‌ Crawl Spaces only have one layout: two ladders and an item, chest, trinket, or various pickups.

These rooms are viewed from the side, rather than from above like the rest of the game, and Isaac will experience gravity pulling him to the floor while he is not walking on top of one of the ladders. Under the effects of flight, this gravity has no effects. However, Isaac cannot fly over the walls or floor.

Mechanics
When entering a room pre-determined to contain a Crawl Space entrance, an RNG roll is made using the floor seed. This value will be shifted and divided, until its final result is in the grid range for the current room (ie: 128 tiles for 1x1 rooms, or 16x8). If said tile value is tied to a simple rock (not tinted, not a prop), then access to the Crawl Space will be granted upon destroying that rock. Only one room per floor has the right to spawn a Crawl Space, and not every floor is given one during the initial generation, even though every floor has also a Crawl Space and a Black Market ready in queue. The tile value can also be tied to props (empty tiles, doors, pots, void tiles, spikes, metal blocks, ...), but the Crawl Space won't spawn unless it's below a simple rock.


 * Crawl spaces may spawn under variants of rocks.

Trivia

 * The layout of the Crawl Space is a direct visual reference to the treasure rooms in the Legend of Zelda for the NES.
 * Crawl Spaces are referred to within the game files as "Item Dungeons".

Seeds
Q16B 7688 (Southeast corner after starting the first wave)