Thin ice acts similarly to a trapdoor, but is more fragile.
Thin ice can exist over deep and shallow water and will sink once the player or mimic steps off of it. Unlike trapdoors, however, thin ice will sink even if the player is wielding a dagger, or even has no weapon at all. The only other creature that can also sink thin ice is construct.
Besides being placed in a room by the architect, thin ice can also form when a puff dies over a tile of water or shallow water.