Falling occurs whenever a non-flying, non-levitating creature finds themselves without a floor beneath their feet. Empty tiles may be described in-game as either chasms or holes depending on the circumstances, but both look and act the same. Causes for falling include:
- Deliberately stepping into a visible chasm tile. You will be given a warning prompt first if you attempt to do this. Allies will follow you into the chasm provided that their hitpoints are high enough.
- Accidentally stepping onto a concealed chasm tile. This is usually a concealed pit trap, but being darkened or wearing a negatively enchanted Ring of Clairvoyance can both prevent you from seeing chasm tiles that are right next to you, allowing you to wander over the edge without warning.
- A chasm opening under the creature due to a Potion of Descent, which will create a temporary area of chasm around where it was opened or thrown.
- If you entrance a creature, you can make it step into a chasm.
Falling causes 2 to 12 points of damage, with a clumping factor of 2. This means that even level 1 characters (15 HP) can survive falling provided that they are at full hit points, but they will be severely injured and vulnerable afterwards. Guardians and Mirror Totems never survive falling, however. The reason for this given in the source code is that they might make the level below impossible to navigate.