Starting tiles, Straight Passages, 2 of the “T” tiles, and Key tiles all show thin crack lines around them. These are Crumbling tiles.
Crumbling tiles collapse into Pits the turn AFTER you step on them. When a Pit forms, flip the tile over to reveal the Pit graphic on the back.
If you decide to Move, it collapses as you scramble onto the next tile.
If you decide to Stay, you Fall into the Pit this turn.
Pits remain until they are no longer Illuminated, and are then removed as normal.