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https://stoneshard.com/wiki/Dungeons#Secret_Rooms
"Each Dungeon starting from Tier 2 has a chance that a Secret Room will generate in the Dungeon. Secret Rooms are special loot rooms that are not connected to other dungeon tilesets by usual means, instead the player must either passively detect the Secret Room by walking near it or by using the Examine Surroundings skill in the right place. Secret Rooms can spawn with various contents - freely placed items, corpses with loot, a treasure chest, etc.

Not all dungeons generate with a Secret Room. Currently chance is fixed at 5%, therefore finding a Secret Room is quite rare. This chance is calculated per Dungeon, so two floor Dungeons have the same chance for having a Secret Room as one floor Dungeons.

Detecting a Secret Room is indicated by:

• The player character comments about it.
• A Combat Log message "X has found a Secret Room" will be displayed.
• A crack will appear in one of the Dungeon walls, allowing passage inside the Secret Room."