Fake floor

From DoomWiki.org

A fake floor is a map design technique where a sector forms a pit with no textures on its walls. This creates an illusionary floor above the height of the actual floor. Among other uses, this visual oddity can be implemented in a relatively deep pit to hide monsters which are raised to attack the player or in a shallower one to add a deep-water effect.

A notorious joke WAD called Wow uses this effect in a comical way, calling a fake-floor pit the player may fall into an illusio-pit.