Making an invisible floor

Invisible floors is an effect that allows a level designer to put invisible solid sectors on a pit in which the player can walk, simulating flying or walking on mid-air.

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. Notice that this trick could not work in some source ports like EDGE, as the trick is not standard.

Steps: 1. Create a sector inside another sector and make sure they don't share any lines. 2. Check the sector's properties, and make a note of what the sector's number is. 3. Edit all the lines on the inner sector you just made and set all the sector references to the sector's number. Both sides of all its lines should reference it. Done.

Limitations

 * Now I can't remember if the floor of the sector surrounding the pit has to be at higher position than the bridge.

Example

 * [[Media:SKYWALK.WAD | Example of this trick]] (file info).