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:
 * Trace two sectors of different size. They must not connected. The larger one will be defined as the pit, and the other will be the bridge.
 * Define the values for them. It could be required to put the same light level and texture on both.
 * Move the pit sector over the bridge. The pit has to contain completely the pit sector and their lines cannot touch.

Trick completed. You can edit both sectors as long as their lines do not contact.