[quote="TheCount":13gqxfzj]I never noticed any switch, at least in hexen, because I always run to get the Urn hoping not to get crushed, and then go for the exit very quickly. I never even look at that room again.
The switch is there in Hexen, and works for all three characters. Look for it on the map, you'll have to jump down and back up on the uneven floor (be careful of a crevice there), then jump down to a small ledge with the switch on the wall. Pull the chain and the floor and ceiling return to normal.