Non-adjacent monsters with the Submerges flag were able to hide or remain hidden in their current tile, even if in clear LOS of the player. A monster so hiding was indistinguishable from the terrain it occupied. While submerged, the monster could move and remain completely hidden. However, any submerged monster adjacent to the player became immediately revealed. Typically, this was seen when certain water-based creatures hide in water or lava-based creatures hide in lava. However, the trapdoor spider could hide on an ordinary dungeon tile (think of it as burrowing underneath the floor).
- The submerging mechanic was removed in 0.22.
- Prior to 0.18, monsters with this flag could resubmerge if the player managed to escape their notice.