Secret Rooms

Secret Rooms are hidden rooms that are accessed by rolling into a pit with a 'sparkle' in it. Around 1-2 of these rooms are generated every floor. There will be a rope that appears when you fall into a secret room. Using it will bring you to where you entered the room (as in, the door you came in). They all contain either of the following:

Chest Room
A room containing a single Green Chest, which is used to teleport items back home. Interact with the chest and fill it with items; once you close out of the menu, the chest will shut and send your items back to the shop.

Wave Room
A room with a pillar connecting to an empty stone slab. This room allows you to activate Horde Mode, in which you fight waves of enemies for loot. After killing all enemies, you can activate the pillar again instead of opening the chest to upgrade it and fight more waves of monsters.

Pedestal Room
A large pedestal appearing in the middle of the room. A single item will be resting atop it (most often the first edition of that dungeon's book collection that you can usually only get from the last boss) that you can acquire yourself by interacting with the pedestal. It is possible to summon the Wanderer when you collect the item unless you leave another item behind, Indiana Jones-style.

Abandoned Room
A room with a bunch of abandoned minecarts and cracks on the ground, usually containing rare loot often found in chests.

Empty Room
Simply an empty room with nothing but the rope to return you to the dungeon.

Dimensional Rift
This room is signified by a swirl of green slime and a cloud of smoke over a pit in a room. Falling into the pit, you'll find a room with a green portal in it. Entering the portal will teleport you to a floor of the Wanderer Dungeon with monsters matching the strength and culture of the dungeon you started in. At the end of the chain of rooms, you'll fight one of the wanderer dungeon minibosses and successfully killing it will drop a Slime Chest, rewarding you with one of the new amulets or trick weapons (but not blueprints) that are scaled for the level of your current dungeon.

Completion of this extra floor also counts as completion of the floor you were previously on, and will teleport you to the next floor of the dungeon. Eg. if you were on floor 1 when you entered a dimensional rift, completing it will drop you back on floor 2.

Trivia

 * It is possible for secret rooms to be inaccessible. For example, the pit could be surrounded by barriers. This is however not intended and is supposed to be fixed in a future update of the game.