In some pwads, for example in MAP01 of 4Seasons of DOOM, downloaded from the /idgames Archive or from any DOOM related forum, the mapper included a trap where monsters rise out of the floor, with no holes in the floor visible.
This trap exploits an oddity of the DOOM engine when lowered sectors have no sidedefs. Then the engine will render the game sector as if no lowered sectors existed.
To construct such a trap, lower some sectors to the height of the monsters you want to use and add a small additional amount. If the monster chosen is the Former Human, the floor height would be ca. -56. Give those sunken sectors an idtag, here 11.
To trigger the effect of rising monsters, add a linedef with the appropriate action special and a walkover trigger. However, that linedef should be placed in such a manner that it is not tripped accidentally. If that becomes problematic then, perhaps, using a switch would be better.
The speed with which the monsters rise is determined by the action special attached to a linedef.
For the monsters to rise instantly use this action special, which exploits another DOOM oddity:
19 W1 Floort Lower to Highest Floor
or slighly slower
130 W1 Floor Eaise to Next Higher Floor (fast)
For a more measured appearance of the monsters use this action special
118 W1 Floor Raise to Next Higher Floor and select the speed.
The map as designed above has a couple of drawbacks.
When the player walks around the map without triggering the trap, then the monsters wake up and start shooting at the player, giving away that there is a trap. Another drawback is that the player will bump ever so slightly on top of the monsters when walking over those sectors.
An alternate approach, to remedy this situation, would be to make the sunken sectors very deep, for example -512, or the sectors very small so that the monsters are encased and cannot move. Or better still, do both.
This trap also works in other game ports, not just in DOOM/BOOM, with the selection of the correct action special, from instant to really slow.