Making superposed sectors

Deep Water is an effect that allows a level designer to put false floor textures on a pit or trench in which the player can sink. If the pit is not very deep, the false floor texture will still be displayed after the player falls, simulating swimming or the player crawling on the floor.

Implementation
There are several methods to obtain this effect. The following is included in Bug's Revolutionary Techniques for Doom and Doom II v2.3 guide, which has defined textures for the bottom of the pit. Notice that this trick could not work in some source ports like EDGE, as the trick is not standard.

Steps:
 * Trace a big enough sector to contain two identical copies of the pool sector. The first one will be defined as the bottom of the pool, and the second will be the surface.
 * Define the values for the surface. The only requirement is that the floor height has to be the same of the surrounding sector. It is a good idea to put the same light level, though, and also F_SKY1 as the ceiling texture.
 * Define the values for the bottom. I suggest a light level of 100-110, using F_SKY1 as the ceiling texture, and some kind of sand as the floor texture. For a pool, a depth of 200 is also recommended.
 * Choose textures for the linedefs of the bottom.
 * On the bottom sector, trace a two-sided linedef from the lower left corner to the upper right corner without texture. If the sector has the shape of a triangle, try adding an extra vertex on it.
 * Move the surface exactly over the bottom, but don't join vertexes.

After this, the trick should be done.

Limitations

 * Objects inside the pool are drawn over the texture. Using a low light level for the bottom of the pool may help to hide some non-luminescent objects inside it.
 * If the floor height of the surface is lower than the surrounding sector, the bottom floor will not have texture. This is caused be the texture being only applied on the walls between the surface and the surrounding sector.
 * If the pool is too depth, the player cannot escape from it.

Example
[[Media:Deep_Water.WAD]] (file info) is an example of this trick. The advantages of using F_SKY1 as the ceiling texture is that you can put different light levels on the pool sectors without the player noticing beforehand, as the ceil does not reflect this. There seems to be a way to avoid this, though, but that would be another trick.