"Swallowing soils"? This does not mean floors that can become invisible or black, but floors that can "swallow" habbos. This looks exactly like this:
When you walk on these floors, you fall into the "nothing". But the floors themselves look quite normal and you can not distinguish them from the others.
The blue cross shows the floor that can swallow habbos. To create such floors you must use the import/export window.
The & sign is displayed in the yellow area. A 0 must be replaced with &. If you save the whole thing, the floor is at a negative height. However, since floors are only displayed from height 0, this floor is visually at height 0. If you step on it, you will fall down.