Also known as the "Reverse Doom Door", this door lowers into the floor when opened, and rises to the ceiling when closed.

Create the door sector and tag it [0,21].

Enter 3D mode and texture the door's frame. You may need to flip the textures around with F to make them look proper. Place an M[StopSound,StartSound] (Speed[0,Speed], S[Delay,10]) in the door sector. I recommend M[0,166].

Point your mouse cursor at the floor of the door sector. Hold the Left Mouse Button to 'lock' the floor, and press PGUP until the floor touches the ceiling. Texture your door, and use the Keypad 2/4/6/8 keys to adjust the texture repeats so they fit properly. Now your door is complete. In the game, you can stand in front of it and press the action button to open and close it.